fix(pcp): compose nested instance proxies

[yohawing]

Summary

Compose nested instance prototypes from the prim that actually composes inside
their enclosing prototype.

Add a three-layer regression fixture containing:

• an instance of a payload-backed outer prim;
• a sub-root payload target;
• a nested instance with a sub-root external reference target; and
• a value below the nested instance proxy.

Root cause

register_prototype accepted an instance path that could itself be an instance
proxy. It built the nested prototype key and materialized root from the sparse
authored namespace at that path. Child-name composition could still discover
the nested reference, but the materialized prototype descendants had no
contributing specs.

Resolve the instance through effective_path first, then use that composed
index for the instance key and prototype materialization. The original path is
still retained for prototype registry membership and load-rule scoping.

Regression fixture

Before this change, nested_payload_proxy fails because
/Placed/Nested/Leaf.value resolves to None. After the change it resolves to
7.0, and /Placed/Nested/Leaf maps to /__Prototype_1/Leaf.

The fixture is intentionally small and contains only three USDA files. Pixar
OpenUSD usdchecker validates its root layer successfully.

Validation

• cargo fmt --all -- --check
• cargo test --all-targets --all-features (2315 passed, 1 ignored)
• cargo clippy --all-targets --all-features -- -D warnings
• Pixar OpenUSD usdchecker on the regression fixture
• Downstream production asset: 2,044 composed Mesh prims and 249 renderable
  Mesh prims, matching Pixar OpenUSD; generated GLB includes the formerly
  missing nested-payload geometry

[mxpv]

Good catch, thanks for opening this!

I think the payload isn't the trigger. I've spent some tokens - minimal repro is a single
layer, no payload — the nested reference just has to target a sub-root prim:

def Scope "Library" { def Scope "Inner" { def Scope "Leaf" { double v = 3.0 } } }
def Scope "Outer" { def Scope "Nested" ( instanceable = true; references = </Library/Inner> ) {} }
def Scope "A" ( instanceable = true; references = </Outer> ) {}

/A/Nested/Leaf.v is None before your change, 3.0 after. Point it at a
root-level </Inner> and it passes without the fix — that's the existing
instancing_nested_in_prototype.usda.

I'll fold in a few cleanups before merging: single-file fixture to match the
other instancing_* ones, the sharing assertions from
nested_prototype_proxy_redirects (same prototype from both routes, exactly 2
minted, proxy never indexed), and materialize_prototype's param, which is
passed the composed path but still named canonical.