refactor(04): move rank.test.ts into tests/ mirror dir (convention + dist hygiene)

src/lib/rank.test.ts was the last co-located API test. The API tsconfig excludes
tests/ from the build, so all test files belong there; a test in src/ gets compiled
into dist/ and vitest then runs the stale compiled copy (the source of phantom
'22 todo' and dist sourcemap warnings). Moving it to tests/lib/ matches convention
and leaves zero test files in src/, so the production build no longer emits test
artifacts. Updated rank.test.ts path refs in phase-04 plan docs.

apps/api/tests/lib/rank.test.ts

@@ -2,11 +2,11 @@
  * Unit tests for rank.ts — fractional-indexing helpers (D-13).
  *
  * Pure function tests, no DB needed.
- * Run: pnpm --filter @familysync/api exec vitest run src/lib/rank.test.ts
+ * Run: pnpm --filter @familysync/api exec vitest run tests/lib/rank.test.ts
  */
 
 import { describe, it, expect } from 'vitest'
-import { rankForAppend, rankBetween } from './rank.js'
+import { rankForAppend, rankBetween } from '../../src/lib/rank.js'
 
 describe('rankForAppend', () => {
   it('returns "a0" when list is empty (no existing rank)', () => {