Extract cn() into src/lib/cn.ts

[BigSimmo]

Summary

• Move cn() from src/components/ui-primitives.tsx to src/lib/cn.ts so the className helper is a lib module rather than a UI-primitives export.
• Re-export cn from ui-primitives so existing importers do not have to move.
• Pin the new location with a source contract: the implementation lives in src/lib/cn.ts, and primitives re-export the same function from @/lib/cn.
• No Button/Sheet migration.

Verification

• npm run verify:pr-local

Arbiter: RUN "verify:pr-local" — CI does not re-run verify:pr-local for this change — local is the only gate.

Decisive lines:

• Test Files 3 passed (3) / Tests 135 passed (135) for tests/ui-primitives.dom.test.tsx (12), tests/ui-v2-components.dom.test.tsx (89), and tests/tailwind-merge-config.test.ts (34, including the new cn module location pin).
• Full suite inside verify:pr-local: Test Files 894 passed (894) / Tests 10830 passed | 1 skipped (10831).
• All matched files use Prettier code style!
• [gate-receipts] recorded a pass for "lint:internal"
• [gate-receipts] recorded a pass for "typecheck:internal"
• ✓ Compiled successfully in 45s
• Offline RAG fixture and manifest validation passed (36 golden cases, 26 suites).
• PR-local verification summary: completed check:runtime, check:installed-lock-parity, format:changed, lint, typecheck, test, check:repo-awareness-snapshot, build, check:rag:fixtures, check:medication-interactions, check:medication-lexicon-report; failed (none).

During development, use npm run verify:cheap as the faster iteration gate before the final PR-local preflight.

• UI verification not run: module-location change only; no visual, routing, or interaction behavior change.
• Release verification not run: verify:release is out of scope for this PR.

For retrieval, ranking, selection, chunking, source/citation rendering, or answer-contract changes, verify:pr-local runs eval:rag:offline automatically. Run the offline command directly during iteration before spending a live eval.

• Retrieval/ranking eval not run: this change does not touch retrieval, ranking, selection, chunking, or scoring.
• Answer-generation eval not run: this change does not touch answer generation, the synthesis prompt, or answer post-processing.
• Production-readiness not run: this change does not touch clinical workflow, privacy, environment, Supabase, source governance, or deployment behavior.
• Deployment-readiness not run: this change does not touch deployment startup, hosting, or rollout behavior.

Risk and rollout

• Risk: Low. cn() behavior is unchanged; ui-primitives still exports the same function. Existing cn() contract tests continue to import from @/components/ui-primitives.
• Rollback: Revert this commit. Importers keep using @/components/ui-primitives.
• Provider or production effects: None
• RAG impact: none

Clinical Governance Preflight

Not required: this change does not touch ingestion, answer generation, search/ranking, source rendering, document access, privacy, production env, or clinical output.

Notes

PR 3d only — extract cn. No other primitives split.

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📥 Commits

Reviewing files that changed from the base of the PR and between 902ccee and 3518065.

📒 Files selected for processing (3)

• src/components/primitive-recipes/recipes.ts
• src/lib/cn.ts
• tests/tailwind-merge-config.test.ts

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