Siderust v0.16.0

[0.16.0] - 2026-10-27

Added

• Generic celestial-body surface observers and geometric Alt/Az transforms:
  SurfaceSite<B>, ReferenceSurface, BodySurfaceModel,
  SurfaceObserver<B>, and coordinates::transform::surface_horizontal
  (position_to_horizontal, direction_to_horizontal, inverse helpers,
  SurfaceHorizontalExt). One ENU pipeline serves Earth, Moon, Mars, and
  other rocky bodies with documented reference surfaces; gas giants omit
  BodySurfaceModel unless a mission supplies an explicit surface.
• First-class lunar and Martian surface Alt/Az using existing
  Selenocentric / MoonIauMeanEarth (legacy MoonPrincipalAxes) and
  Marscentric / MarsFixed orientation providers (IAU WGCCRE).
  Finite-distance parallax is applied only to positions; direction-only
  inputs do not invent a range.
• Explicit east-positive longitude storage with
  PlanetographicLongitudeSense, east_positive_from_planetographic, and
  SurfaceSite::from_planetographic_longitude for NAIF SPICE interoperability
  (Mars/Mercury/Pluto planetographic longitudes are west-positive).
• Composite lunar Alt/Az validation: CSPICE IAU_MOON orientation plus
  independent ENU geometry, with regeneration script
  tools/cspice_refs/cspice_surface_references.py.
• Scale-free zenith/nadir azimuth singularity (32 ε) so exact vertical
  lines of sight report the documented 0° azimuth convention without
  inventing noise-driven bearings.
• Preferred MoonIauMeanEarth position aliases alongside the legacy
  MoonPrincipalAxes names; documentation no longer describes the analytical
  model as a true principal-axes frame.
• Default surfaces with documented provenance: WGS84 (Earth), IAU mean
  sphere 1737.4 km (Moon), IAU ellipsoid a = 3396.19 km (Mars), plus Mercury,
  Venus, and Pluto mean spheres. Planetocentric vs planetographic latitude
  conventions are explicit.
• SurfaceObserverParams::try_origin as the fallible counterpart to Default
  (affn still requires Params: Default; the trait contract is that
  default_surface admits an origin site).

Changed

• Surface sites are parameterized directly by solar-system body markers
  (SurfaceSite::<Moon>, SurfaceSite::<Mars>, …). Redundant pre-release
  *Surface type aliases were removed.
• BodySurfaceModel no longer exposes runtime has_reference_surface /
  ensure_supported checks; unsupported bodies simply do not implement the
  trait.
• Observer-tagged horizontal direction results through SurfaceHorizontalExt;
  observer parameters now carry the body at the type level, preventing
  accidental site and body mismatches in checked inverse transforms.
• Examples 21_body_surface_horizontal and beginner-friendly
  22_moon_surface_quickstart, plus offline scientific validation
  tests (tests/body_surface_horizontal.rs) covering zenith, cardinals,
  poles, parallax, epoch rotation, forward/inverse round-trips, and
  invalid inputs.
• EquatorialMeanJ2000 ↔ planetary body-fixed frame rotations via the ICRS hub.
• Add CITATION.cff for scholarly software citations and document how to
  cite Siderust in the README.

Fixed

• Correct the pre-existing IAU body-fixed → ICRS active ZXZ rotation
  convention; pin the Moon/Mars poles and prime meridians to independent
  J2000 reference vectors.
• Bring the lunar analytical IAU_MOON orientation in line with the NAIF
  text-PCK: all 13 periodic arguments and the quadratic term in W, with
  multi-epoch pole and prime-meridian vector regression cases.
• Use a consistent east-positive Cartesian Horizontal Y axis in both new
  surface and legacy Earth-only transforms, matching affn's spherical
  azimuth semantics (north through east). Previously Cartesian azimuth was
  mirrored against spherical azimuth.
• Make the ellipsoid surface normal the geometric vertical for both
  planetocentric and planetographic site representations; height is measured
  along that normal in either convention.
• Validate deserialized reference surfaces, sites, and observer parameters;
  permit subsurface heights up to the documented normal-coordinate singularity.
• Stabilize small-angle round-trip assertions without loosening tolerances;
  reject zero/non-finite inverse directions; preserve range for extremely
  distant targets and scale-independent normals for tiny/large ellipsoids.
  Reject requested units that overflow or erase an observer position.
• Convert lunar orientation epochs from TT to TDB using tempoch and validate
  all three axes against offline CSPICE references spanning 1900--2050.

Changed

• Synchronize siderust and unpublished siderust-ffi metadata at 0.16.0,
  including the internal dependency and C ABI version 1600.
• Document generic surface observers in doc/frames_and_centers.md and
  doc/architecture_decisions.md. Earth Topocentric / GAST horizontal
  APIs remain unchanged for backward compatibility.

Notes

• Geometric Alt/Az only: no refraction, aberration, light deflection,
  light-time, or terrain masking in the new path.
• The backward-compatible MoonPrincipalAxes frame marker now uses
  NAIF's analytical IAU_MOON (IAU 2009) Mean-Earth/Polar-Axis orientation;
  MoonIauMeanEarth is a clearer alias for this implementation. It is
  not JPL DE MOON_PA, nor DE-specific binary-PCK MOON_ME.
  The change corrects the orientation of existing MoonPrincipalAxes
  coordinates. Lunar time arguments use TDB, explicitly converted from TT
  by tempoch; this analytical model must not be marketed as DE-level precision.