Experimental import migration

Validated mathematics has one implementation in the supported core. New code should use core imports. The experimental directory still exists: it contains only minimal compatibility reexports and a package marker. Complete removal would break previously documented imports, so the remaining paths await an explicit release-boundary and compatibility-window decision. No runtime warnings or duplicated algorithms are introduced.

Historical import	Canonical replacement	Compatibility status	Removal status / migration action
gem.experimental.bezier	gem.bezier	Identical functions and BezierPath class	Retained shim; update imports
gem.experimental._bezier_legacy.BezierPath	gem.bezier.BezierPath	Identical class; private path remains importable	Retained shim; update imports
gem.experimental.legendre.Legendre	gem.legendre.Legendre	Identical class	Retained shim; update imports
gem.experimental.sph	gem.spherical_harmonics	Identical Factorial, K, SPH; incidental Legendre retained	Retained shim; prefer gem.legendre for Legendre
gem.experimental.sph_sample	gem.spherical_harmonics	Identical SPHSample, GenerateSamples	Retained shim; update imports
gem.experimental.sph_irradiance_map	gem.spherical_harmonics	Identical SPH_IrradianceMapCoeff	Retained shim; update imports; legacy basis still requires explicit conversion
gem.experimental.sph_object	None	Unfinished transport retired	Removed; SPHVertex, SPHObject, GenereateCoeffs imports fail
gem.experimental	Core modules above	Package marker retained for shims	Not fully removed; no new experimental implementations

E07 never provided implemented visibility transport. Its removal is an import break for consumers of its containers or unfinished routine; it is not a mathematical correction or a successful replacement of shadow transport. Repository consumers consisted of its expected-failure and diagnostic tests. External usage is unknown. Historical source remains in git; the original test and nine diagnostic cases are archived under audit/retired-tests, with fault results in audit/PHASE2F6.md. They are deliberately retired from collection, not skipped or relabeled as fixed. New tests assert this module's absence.

The retained paths continue to ship in both wheels and source distributions. Changing an import to its core equivalent preserves signatures, storage, ownership, basis signs, angle units and numerical behavior. Updating the import does not convert historical probe coefficients into canonical SH: use legacy_to_canonical explicitly. Existing serialized references to retired transport classes may require application-specific migration; no serialization shim is provided.

Future package deletion must choose a release boundary, announce all removed paths, migrate compatibility tests and change packaging together. Scene visibility, acceleration and transport remain outside core math. No new transport API or Phase 3 optimization is included.