test: undo/redo state integrity + ensureDataIds idempotency tests

[cheerc]

What

Add comprehensive tests for undo/redo state management (#135) and ensureDataIds idempotency (#137).

Closes #135
Closes #137

How

• DI extraction (Strategy 1): Extracted createHistoryModule undo/redo stack logic from History.js.html L2-62 into tests/lib/undoRedoHelpers.js, preserving all behavioral contracts (dedup, redo truncation, 50-entry cap, boundary guards).
• DI extraction (Strategy 1): Extracted ensureDataIds + generateUniqueId from JavaScript.html L881-900 into tests/lib/dataIdHelpers.js with injectable ID generator for deterministic testing.
• 27 new undo/redo tests in tests/unit/undoRedoState.test.js covering:
  • saveState: dedup identical consecutive, truncate redo branch, 50-entry cap
  • undo: restore previous state, consecutive undos, boundary guard (index 0)
  • redo: restore next state, consecutive redos, boundary guard (end of stack)
  • interleaved undo/redo cycles, rapid back-and-forth stability
  • resetHistory: clear stack, disable undo/redo
  • canUndo/canRedo: boundary conditions
  • State isolation: structuredClone independence
  • Complex nested scheduleData through undo/redo round-trips
• 23 new ensureDataIds tests in tests/unit/ensureDataIds.test.js covering:
  • Adding IDs to items without them (single, multiple, cross-classroom)
  • Idempotency: existing IDs preserved, multiple calls yield same result
  • Mixed partial IDs across classrooms and days
  • Edge cases: null, undefined, empty, non-array day values, null items
  • ID format: non-empty string, uniqueness, base-36 characters
  • Mutation behavior: returns same reference, mutates in place
• Wiring contract update: module count 13 → 15

Scope

• Test-only changes — zero production code modifications
• Files changed: tests/lib/undoRedoHelpers.js (new), tests/lib/dataIdHelpers.js (new), tests/unit/undoRedoState.test.js (new), tests/unit/ensureDataIds.test.js (new), tests/unit/wiringContracts.test.js (count update)

Verification

• ./workflow.sh t6 — 8/8 PASS (t1-t5, t7-t9)
• npx vitest --run tests/unit/undoRedoState.test.js tests/unit/ensureDataIds.test.js — 50 tests passed
• Scope gate: git diff --name-only origin/main...HEAD shows only test files

follows spec

[cheerc]

✅ VERIFIED

Reviewer: cb-team-reviewer
Reviewed HEAD: 7e2f6eb753da279f475b6d64dd28bb5da319a4b2
Audit mode: fast_path (D1, correctness)
Skills: review-task-loop, ecc-code-reviewer

Files Reviewed

• tests/lib/undoRedoHelpers.js (107 lines, NEW) — DI extraction of undo/redo stack from History.js.html L2-62
• tests/lib/dataIdHelpers.js (53 lines, NEW) — DI extraction of ensureDataIds from JavaScript.html L881-900
• tests/unit/undoRedoState.test.js (413 lines, 27 tests) — saveState/undo/redo/resetHistory/canUndo/canRedo/state isolation/complex scheduleData
• tests/unit/ensureDataIds.test.js (255 lines, 23 tests) — ID assignment/idempotency/mixed partial/edge cases/ID format/mutation
• tests/unit/wiringContracts.test.js (6 lines ±) — module count 13 → 15

Summary

Pure test-only PR (zero production code changes). 50 new tests across two DI-extracted helper modules.

undoRedoHelpers.js: Extracted createHistoryModule stack logic as createTestableHistoryModule with injected callbacks. Preserves all behavioral contracts: dedup via JSON.stringify comparison, redo truncation on new action, 50-entry cap with shift, boundary guards (index > 0 for undo, index < length-1 for redo).

dataIdHelpers.js: Extracted ensureDataIds + generateUniqueId from JavaScript.html L881-900. Verified line-by-line against source. Only DI change: this.generateUniqueId() → injected idGenerator parameter.

undoRedoState.test.js (27 tests): saveState (dedup, truncate redo, 50-cap, shift oldest, callbacks), undo (restore, consecutive, boundary guard), redo (restore, consecutive, boundary), interleaved cycles (full undo/redo traversal, rapid back-and-forth), resetHistory (clear + disable undo/redo + callbacks), canUndo/canRedo boundary conditions, state isolation (structuredClone independence), complex nested scheduleData round-trips.

ensureDataIds.test.js (23 tests): ID assignment (single, multiple, cross-classroom), idempotency (existing preserved, multiple calls identical, three consecutive), mixed partial IDs, edge cases (null/undefined/empty/0 input, empty data, no days, empty day array, null classroom, non-array day, null items), ID format (non-empty, uniqueness ×100, base-36 chars), mutation behavior (same reference, in-place).

Stage 1 — Correctness

Zero findings. No conflict markers. Imports resolve. Issue linkage (Closes #135, #137). No risk-flags triggered. Both DI extractions verified against source.

Stage 2 — Adversarial

Zero findings. Dedup uses JSON.stringify comparison (matches source — same limitation: key order dependent, but consistent within same app). State isolation test verifies structuredClone independence correctly. createTestContext helper cleanly simulates app state mutation via closured appState. 50-entry cap test correctly verifies shift behavior (Room-2 at index 0 after 52 saves). Deterministic idGenerator injection enables reproducible assertion without time-dependency.

Evidence

• ran: gh pr view 143 --json headRefOid → 7e2f6eb753da279f475b6d64dd28bb5da319a4b2 (SHA aligned)
• ran: gh api .../JavaScript.html L881-900 → ensureDataIds source verified: this.generateUniqueId() → injected idGenerator
• ran: gh api .../dataIdHelpers.js | grep conflict-markers → 0
• cited: dataIdHelpers.js:L39-50 — ensureDataIds traversal matches source L881-896 exactly (null guards, Array.isArray check, classItem && !classItem.id)
• cited: undoRedoHelpers.js:L47-48 — dedup via JSON.stringify(currentState) === JSON.stringify(history[historyIndex]) matches History.js.html original
• cited: undoRedoState.test.js:L79-83 — 50-entry cap: history.shift(); historyIndex-- after 55 saves → stack[0] = Room-2