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

@@ -0,0 +1,122 @@
+/**
+ * Unit tests for rank.ts — fractional-indexing helpers (D-13).
+ *
+ * Pure function tests, no DB needed.
+ * Run: pnpm --filter @familysync/api exec vitest run tests/lib/rank.test.ts
+ */
+
+import { describe, it, expect } from 'vitest'
+import { rankForAppend, rankBetween } from '../../src/lib/rank.js'
+
+describe('rankForAppend', () => {
+  it('returns "a0" when list is empty (no existing rank)', () => {
+    const rank = rankForAppend(null)
+    expect(rank).toBe('a0')
+  })
+
+  it('returns a rank string that sorts AFTER the given last rank', () => {
+    const lastRank = 'a0'
+    const newRank = rankForAppend(lastRank)
+    expect(newRank > lastRank).toBe(true)
+  })
+
+  it('multiple appends produce strictly increasing ranks', () => {
+    let last: string | null = null
+    const ranks: string[] = []
+    for (let i = 0; i < 5; i++) {
+      const r = rankForAppend(last)
+      ranks.push(r)
+      last = r
+    }
+    // Each successive rank must be greater than the previous
+    for (let i = 1; i < ranks.length; i++) {
+      expect(ranks[i] > ranks[i - 1]).toBe(true)
+    }
+  })
+})
+
+describe('rankBetween', () => {
+  it('returns "a0" when both prev and next are null (empty list)', () => {
+    const rank = rankBetween(null, null)
+    expect(rank).toBe('a0')
+  })
+
+  it('returns a rank that sorts between two existing ranks', () => {
+    const first = rankForAppend(null)    // 'a0'
+    const second = rankForAppend(first)  // 'a1'
+    const between = rankBetween(first, second)
+    expect(between > first).toBe(true)
+    expect(between < second).toBe(true)
+  })
+
+  it('returns a rank that sorts AFTER prev when next is null', () => {
+    const prev = 'a0'
+    const rank = rankBetween(prev, null)
+    expect(rank > prev).toBe(true)
+  })
+
+  it('returns a rank that sorts BEFORE next when prev is null', () => {
+    const next = 'a1'
+    const rank = rankBetween(null, next)
+    expect(rank < next).toBe(true)
+  })
+
+  it('produces lexicographically stable ordering across multiple insertions', () => {
+    // Simulate inserting between 'a0' and 'a1' repeatedly
+    const a = 'a0'
+    const b = 'a1'
+    const c = rankBetween(a, b)
+    const d = rankBetween(a, c)
+    const e = rankBetween(c, b)
+
+    // All four ranks should be orderable
+    expect(a < d).toBe(true)
+    expect(d < c).toBe(true)
+    expect(c < e).toBe(true)
+    expect(e < b).toBe(true)
+  })
+
+  /**
+   * Precision regression test — LIST-03, Pitfall 2.
+   *
+   * Repeated "zipper" inserts (always between the first two items) must produce
+   * strictly increasing, unique, non-empty rank strings across many iterations.
+   * Float-based approaches would exhaust precision around iteration 52; the
+   * fractional-indexing string approach degrades gracefully by growing string
+   * length instead.
+   */
+  it('repeated mid-point inserts produce unique strictly-increasing ranks over 100 iterations (precision — Pitfall 2)', () => {
+    const ranks: string[] = [rankBetween(null, null), rankBetween(null, null)]
+    // Set up: two items with known ranks
+    ranks[0] = rankBetween(null, null)         // 'a0'
+    ranks[1] = rankBetween(ranks[0], null)     // 'a1'
+
+    // Insert 100 times between the first item and the second item
+    // This is the worst-case "zipper" pattern — always inserting at the same gap
+    for (let i = 0; i < 100; i++) {
+      const newRank = rankBetween(ranks[0], ranks[1])
+      // Must be strictly between
+      expect(newRank > ranks[0]).toBe(true)
+      expect(newRank < ranks[1]).toBe(true)
+      // Must be a non-empty string
+      expect(newRank.length).toBeGreaterThan(0)
+      // Must be unique (not equal to any existing rank)
+      expect(ranks).not.toContain(newRank)
+      // New item goes at index 1 (after first, before old second) — shift old items
+      ranks.splice(1, 0, newRank)
+    }
+
+    // Verify the final list is fully sorted ASC
+    for (let i = 1; i < ranks.length; i++) {
+      expect(ranks[i] > ranks[i - 1]).toBe(true)
+    }
+  })
+
+  it('rank between two neighbors is strictly between them (D-13 reorder contract)', () => {
+    const prev = 'a0'
+    const next = 'a3'
+    const middle = rankBetween(prev, next)
+    expect(middle > prev).toBe(true)
+    expect(middle < next).toBe(true)
+  })
+})