feat(core): rule color/palette-conformance (Oklab ΔE00)

[aram-devdocs]

Summary

Match rendered colors against color.tokens using Oklab ΔE00
with delta_e_tolerance (default 1.5). Blend semi-transparent
colors against the CDP-reported blended background.

Crate / area

plumb-core — rule engine, types, determinism (no internal deps)

Phase / batch

Part of #20 → Batch 2B (Remaining MVP rules (5 parallel implementations)).

Effort: L

PRD references

• PRD §11.2
• PRD §11.3

Acceptance criteria

• Rule flags off-palette colors within tolerance.
• Blended-background semantics match PRD §11.3.
• Palette converted to Oklab once at config load.
• Golden test + docs + register_builtin.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-core/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue