v0.5.0

openusd continues to gain usable shape and form. This release introduces
high-level APIs to manage stages, layers, primitives, and more — while
bringing several major subsystems to spec compliance.

• The PCP composition engine is fully rewritten around a task-queue indexer
  that now conforms to all pcp.txt compliance tests, covering correct
  LIVRPS strength ordering, scene-graph instancing, specializes, relocates,
  value clips, and arc permissions.

• Layer management graduates from a flat stack to a full LayerGraph model
  with surgical change invalidation. Stage gains a new set of APIs to add,
  remove, and mute layers.

• Scene-query APIs move from Stage onto the Prim, Attribute, and
  Relationship handles where they belong.

• All domain schemas (geom, lux, shade, render, skel, physics,
  vol, ui, proc, media) are rewritten as composable trait-based
  views modelling the schema object hierarchy with supertrait chains. The
  same handle exposes both read and author operations, and setters chain:

  use openusd::{usd, schemas::geom};
  
  let stage = usd::Stage::open("scene.usda")?;
  for prim in stage.traverse() {
      if let Some(mesh) = geom::Mesh::new(prim.clone()) {
          // read
          let counts = mesh.face_vertex_counts_attr()
              .get::<Vec<i32>>(usd::TimeCode::DEFAULT)?;
          // author
          mesh.create_extent_attr().set(extent_value)?;
      }
  }

  The trait hierarchy mirrors the C++ schema hierarchy —
  SchemaBase → Imageable → Xformable → Boundable → Gprim → PointBased → Curves
  — with concrete newtype views like Mesh, Camera, and Sphere.

• Math utilities scattered across the crate are consolidated into a new gf
  module mirroring the C++ Gf namespace: Vec2/3/4, Quat, and Matrix
  types in multiple numeric precisions, all bytemuck::Pod for zero-copy
  binary serialization.

New AOUSD Core Spec coverage

New AOUSD Core Spec coverage in this release:

• 7.6.1.2.2 Layer-offset retiming applied during value resolution
• 8.2 Element ordering (sdf::element_cmp, primOrder / propertyOrder reordering)
• 10.3.3 Arc permissions — direct arcs to permission = private sites are inerted and reported as pcp::Error::ArcPermissionDenied
• 11.3.1 Ordered prim children via primOrder reordering
• 11.3.2 Ordered property children via propertyOrder reordering
• 11.3.3 Scene-graph instancing — instances share one composed prototype; nested instances, instance-proxy redirects, and population-mask integration covered
• 11.5 IN_PROTOTYPE flag and instance-proxy traversal toggle
• 12.3 Basic attribute resolution — layer-offset retiming applied during value resolution
• 12.3.4 Value clips — explicit and template clip resolution, manifest gating, active-clip selection, stage-to-clip time mapping, interpolateMissingClipValues
• 12.4 Relationship targets (raw + forwarded chains) and attribute connections with a stage-wide connection graph
• 15.1 / 15.2 CollectionAPI — apply_collection, include_path / exclude_path, MembershipQuery, compute_included_paths

PCP composition engine

• Rewrote the composition engine as a priority task-queue indexer (Indexer)
  mirroring C++ Pcp_PrimIndexer, now the sole composition path (abf6ca4)
• Replaced LayerStack with a full LayerGraph model: LayerNode,
  LayerGraph, and pcp::LayerId handles with O(1) identifier lookup;
  sublayer edges derive from authored metadata and are rebuilt on change
  (f2eab23)
• Implemented LIVRPS arc strength ordering via #[repr(u8)] ArcType and
  the compare_sibling_node_strength / compare_node_strength comparators
  (095728a)
• Graph-clone seed for ancestral opinions: a child prim clones its parent's
  composed graph and deepens every site path, so referenced ancestor arcs
  re-evaluate at the child (4ea2ae4)
• References and payloads compose as per-(layerStack, path) site nodes,
  retaining culled arc targets as CULLED nodes for structural visibility
  (a714040, c27146f)
• Inherits and implied classes including subroot-arc propagation (5773348,
  a188515, c80e421)
• Specializes as globally-weak class-based arcs with propagate_node_to_root
  and implied-specializes propagation (f6f5fe7, 141c610)
• Local and ancestral variant sets resolved through a second task band;
  EvalNodeVariantAuthored → EvalNodeVariantFallback with live-node
  strength tie-breaking (40bb677, 7c4de1f)
• Expression-valued asset paths in reference / payload arcs evaluated during
  index build; composed_expr_vars folds expressionVariables across arcs
  (41ab8a8, 7119565)
• MapFunction invertibility-checked target translation,
  without_variant_selections stripping, and true compose semantics
  (0638cea, b04dffe)
• Relocates ported into the task-queue indexer via eval_node_relocations;
  per-layer layerRelocates validated and chained within a node's layer
  stack (0b642d3, 65fbf28, 4484d7d)
• Direct arcs to permission = private sites inerted and reported via
  pcp::Error::ArcPermissionDenied; descendant arcs inerted through
  denied_prefixes (9ae8510, a6a5b93)
• Scene-graph instancing: PrototypeRegistry keyed by InstanceKey
  (arc + variant selections); prototype namespace composed in place, instance
  proxies redirect onto /__Prototype_N; targeted invalidation via
  invalidate_prototypes (cc43d29, 1285318, e659568)
• Value clips: ClipSet parsing, manifest gating, active-clip selection,
  stage-to-clip time mapping, template clip expansion, and
  interpolateMissingClipValues (d6d4d83, 14c93fc, a837749)
• Layer-offset retiming of timeSamples during value resolution (8886672)
• Relationship target forwarding through chains with cycle-breaking (737fdd1)
• Attribute connection graph: ConnectionGraph stage-wide index with composed
  connectionPaths and authoring on Attribute (d75feb7, c864da8)
• Dependencies reverse map for surgical change fanout
• Changes / IndexCache::process_changes for three-tier change
  classification and targeted cache invalidation
• Recoverable composition errors collected via Stage::composition_errors
  rather than callbacks (750af7e)
• PrimIndex::dump_to_string composition-tree dump for debugging (e668aea)
• reorder rootPrims parsed and emitted in USDA (0543612)

Stage and layer management

• LayerGraph replaces the flat LayerStack; Stage::insert_sub_layer /
  remove_sub_layer and sub_layers routed through it so edits persist on
  save (f2eab23)
• Direct access to the session layer from Stage (18afede)
• Stage::mute_layer and layer muting support
• StageBuilder builder methods shortened to load / mask (8d7fb2b)
• EditTarget and EditContext (RAII guard) for routing authoring through
  any layer; for_local_direct_variant for variant-namespace writes
• Stage refactored as a cheap Rc<StageInner> reference-counted handle —
  cloning bumps the refcount, no lifetime parameters on Prim / Attribute
  / Relationship (700e2c2)

Schema views

• usd::SchemaBase trait introduced as the schema-view foundation (665467a)
• UsdGeom migrated: Imageable → Xformable → Boundable → Gprim → PointBased → Curves
  chain, all intrinsic shapes, Camera, Xform/Scope, Mesh +
  GeomSubset, curve/point/patch types, PointInstancer (2ff1faa)
• UsdLux migrated: light hierarchy and LightAPI / ShapingAPI applied
  API schemas (cede873)
• UsdShade migrated: Connectable interface trait, Shader / NodeGraph /
  Material, MaterialBindingAPI with direct + collection binding resolution,
  UsdPreviewSurface + UsdUVTexture (b05b4f3 … f7c35ab)
• UsdRender migrated: RenderSettingsBase / RenderSettings /
  RenderProduct / RenderVar / RenderPass / RenderDenoisePass with
  compute_render_spec (f6f8d71 … 10be3a1)
• UsdSkel migrated: SkelRoot / Skeleton / SkelAnimation /
  BlendShape, SkelBindingAPI, SkeletonResolver / SkinningResolver,
  LBS + blend-shape math (fd39231)
• UsdPhysics migrated: typed Scene / Joint prims, single- and
  multi-apply API schemas (DriveAPI / LimitAPI) (ed641fb)
• UsdVol migrated: Volume + OpenVDBAsset / Field3DAsset (7780720)
• UsdMedia migrated: SpatialAudio, AssetPreviewsAPI thumbnails (5ebf1b1)
• UsdProc migrated: GenerativeProcedural (e6883c1)
• UsdUI migrated: Backdrop, SceneGraphPrimAPI, NodeGraphNodeAPI
  (042519c)
• Attribute::get made generic; Value From/TryFrom impls for all gf
  types (a95f6f5, 97133b1)
• Typed token ↔ Value conversions via impl_token_value! macro (97133b1)
• CollectionAPI — apply_collection, include_path / exclude_path,
  MembershipQuery, compute_included_paths (80285f6 … 404531a)

Resurfaced API — moved to handles

• Prim::prim_stack, Prim::prim_index, Prim::children,
  Prim::property_names, Prim::attributes, Prim::relationships,
  Prim::variant_sets (a3fb43c, 8ea96c8)
• Attribute::property_stack, Attribute::connections,
  Attribute::compute_connections (a3fb43c)
• Relationship::property_stack, Relationship::targets,
  Relationship::forwarded_targets, Relationship::compute_targets (a3fb43c)
• Prim::set_metadata for prim-level fields (7d5c02f)
• Attribute and Relationship split into their own modules (209f441)

Graphics foundations (gf)

• Vec2/3/4 in f32 (f), f64 (d), f16 (h), and i32 (i)
  variants; Quatf/d/h with (w, x, y, z) field order and slerp;
  Mat2d, Mat3d, Matrix4d (row-major) (5d5b0db)
• All types are #[repr(C)] / #[repr(transparent)] + bytemuck::Pod
  for bulk-cast binary serialization
• Free-function constructors (vec3f, quatf, …) and lerp / slerp
  in gf/mod.rs

SDF

• AssetPath value type (sdf::AssetPath / Value::AssetPath /
  Value::AssetPathVec) with asset[] array support (2d0ff3c, e1ca0de)
• sdf::TimeCode value type (5ebf1b1)
• Layer relocates API: relocates / set_relocates / has_relocates /
  clear_relocates (e012c59)
• sdf::element_cmp for USD namespace element ordering (85c2ed3)
• Variant-namespace spec authoring in Layer (ed3b558)
• Path::components and split_property; corrected parent(), prim_path,
  and last_element for edge paths (49c5321, 9fd4b9d)
• AssetPath serde support behind the serde feature (e1ca0de)

USDA / USDC

• reorder rootPrims parsed and emitted (0543612)
• timecode attribute type parsed (8861ce0)
• Variant selections rejected in arc paths (ff3616f)
• asset path and path expression array types in USDC (fb5076c)
• vec3h / vec4h inlined decoding fixed (722bc79)

Dependencies

• codecov/codecov-action bumped from 6 to 7 (28f702d)