core: derive a typed CAM16-UCS occurrence view

[lemone112]

Summary

• Add a typed, lazy rectangular CAM16-UCS view (J′, a′, b′) derived only from an already admitted CAM16 appearance state of the same exact LCS occurrence.
• Keep LCS identity as exact stimulus plus immutable appearance context. CAM16 and CAM16-UCS remain one-way coordinate views; this adds no editable hybrid color space, inverse route, pairwise distance promise, or second context owner.
• Make the CAM16-UCS release identity explicit and register its admitted frame/domain, coordinate units, achromatic law, reference identity, and direct dependency on the canonical CAM16 forward release.
• Preserve optional cost: evaluators which request only Oklab or CAM16 pay no CAM16-UCS logarithm or polar-to-rectangular trigonometry.
• Rebind the fail-closed point-support proof to the exact LCS source bytes; all algebra, certified claims, Q55 dependency, and negative-control count remain unchanged.

Scope

• Exactly 6 files stacked on core: close the evaluator union and package session bridge #454 (3e8f240…), +329/-23.
• No PairFill, PairLabel, Glow, Material, Ladder, AlphaAnalog, client vocabulary, recipe compatibility layer, inverse transform, ΔE/distance calibration, gamut promise, WASM export, controller, or arbitrary resource cap.
• CAM16-UCS is deliberately classified as an appearance view, not a difference calibration.

Contracts

• M′ == 0 produces canonical positive-zero a′ and b′ without consulting an undefined hue.
• Positive M′ with an undefined hue is a typed inconsistent-derived-state failure.
• Every emitted coordinate is finite; J′ and the intermediate M′ are non-negative.
• Registry serialization is canonical and versioned, with the CAM16 dependency encoded explicitly.

Verification

• Rust 1.96 labcolors-core tests: 715 passed, 6 ignored
• Integration suites, doctests, and external package bridge RED contract
• clippy -D warnings
• rustfmt check
• point-support verifier
• proof claims/algebra semantic diff
• exact-head GitHub CI
• exact-head Native conformance
• CodeRabbit

This is a stacked draft. Review only against agent/mixed-evaluator-package-bridge; do not merge independently.

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[lemone112]

Superseded by the reviewed cumulative squash merge #465 (24fd1f4). The lower stacked branch was intentionally not merged on its own because its intermediate head was not the safe terminal public boundary.