fix(collaboration): validate public contrast colors

[seonghobae]

Closes #305

Scope

• reject non-string and malformed public contrast-color inputs before channel parsing
• preserve the existing six-digit hexadecimal grammar and WCAG relative-luminance result for valid values
• reuse the isolated RED/GREEN lineage from closed Draft fix(collaboration): harden awareness runtime contracts #167 on current protected main

Exact-head local evidence

• RED on test-only 28b18ede6008da34a6c671f3b740407b3bdfce31: 5 failed / 1 passed
• focused GREEN: 6 / 6 tests passed
• pnpm test: 157 files / 887 tests passed
• pnpm run coverage: 100% statements, branches, functions, and lines
• pnpm run typecheck: passed
• pnpm run build: passed
• pnpm run verify:package: passed

The existing collaboration documentation already states the six-digit color grammar and relative-luminance behavior, so this issue-specific repair does not change public authority or host/provider ownership. Keep Draft while #118 owns the protected release boundary; no predecessor checks or approvals transfer.

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