test: ratchet coverage thresholds + fill tests/lib branch gaps (#107 P0)

[cheerc]

What

Coverage threshold ratchet + branch gap fill for tests/lib helpers. Part of #107 Phase 2 P0.

Part 1: Threshold Ratchet

Bump vitest coverage thresholds to match current actuals minus ~3% buffer:

Metric	Old	New	Actual
Statements	55%	67%	70.71%
Branches	65%	74%	78.14%
Functions	75%	82%	87.09%
Lines	55%	67%	70.06%

Part 2: Branch Gap Fill

New tests/unit/libBranchGaps.test.js (12 tests) covering:

undoRedoHelpers.js (71.42% → 100% branch) 🎉

• L33-36: Default parameter branches for optional callbacks (onLoadState, onUpdateButtons, onCheckDirty, onUpdateCleanSnapshot)
• Tests: exercise module with omitted callbacks + with provided callbacks

scheduleListHelpers.js (96.66% → 100% branch) 🎉

• L59: backendResult.lastModified falsy branch — backend omits lastModified, scheduleLastModified should not be updated

dataCollectionHelpers.js (92.06% branch — improved but some gaps remain)

• collectFromAllCourses: null schedules, missing data/scheduleData, null classroom, non-array day schedule
• getGlobalAllTags: courses without tags, non-array tags
• getGlobalAllCourseNames: courses without name
• getGlobalAllTeachers: courses without teacher

Changed Files (2)

• vitest.config.js (threshold update)
• tests/unit/libBranchGaps.test.js (new, 12 tests)

Verification

• npm test: 1154 tests passed (37 files) ✅
• npx vitest run --coverage: all thresholds pass ✅
• 2 of 3 target files at 100% branch coverage ✅

refs #107

[cheerc]

✅ VERIFIED

Reviewed HEAD: 31022e6
Reviewer: cb-team-reviewer
Audit mode: fast_path (D1 — test-only, no production code)

Files Reviewed (2)

• vitest.config.js — threshold ratchet 55/65/75/55 → 67/74/82/67
• tests/unit/libBranchGaps.test.js (new, +240) — branch gap tests for 3 lib helpers

Fast Path Checklist

• No production code changes
• Threshold ratchet: ~3% buffer from actuals (70.71/77.38/84.94/70.06) ✓
• New test targets documented uncovered branches:
  • undoRedoHelpers L33-36 (default param no-ops)
  • dataCollectionHelpers L58/77-78 (null guards, non-array tags)
  • scheduleListHelpers L59 (lastModified falsy)
• Correct imports from tests/lib/
• refs [Test-Coverage] Phase 2 — 覆蓋缺口補齊 + 重構前置強化 #107

Evidence

• cited: vitest.config.js — lines:67, functions:82, branches:74, statements:67
• cited: libBranchGaps.test.js:1-9 — targets 3 helpers with documented branch %