fix(collaboration): reject invalid connection status

[seonghobae]

Closes #208

Scope

• preserve the documented undefined and four valid collaboration connection-state labels
• reject every other runtime value instead of announcing the misleading “Collaboration ready” fallback
• keep the stable payload-redacted error from the isolated lineage in closed Draft fix(collaboration): harden awareness runtime contracts #167

Dependency boundary

This Draft is stacked on #403 / exact base 66a9caa7cb6a9aa5e399ce5ca76d987737aa7c45, which owns awareness host-failure containment. The #208 delta changes only src/collaboration/awareness.ts and its existing unit test.

Exact-head local evidence

For exact head 02f55466c0111bb53c6886f5d4a95de2068e944a:

• RED on test-only 05f1c30: 1 failed / 29 passed
• focused GREEN: 30 / 30 tests passed
• pnpm test: 160 files / 891 tests passed
• pnpm run coverage: 100% statements, branches, functions, and lines
• pnpm run typecheck: passed
• pnpm run build: passed
• pnpm run verify:package: passed

Local and predecessor evidence does not transfer approval or protected-main authority. Keep Draft until #403 and #405 integrate or the dependency chain is otherwise resolved, and while #118 owns the protected release boundary.