test: add property-based FSM tests for workflow persistence (#9370)

## Summary

- Fixes #9319
- Add [fast-check](https://github.com/dubzzz/fast-check) property-based
testing with FSM (Finite State Machine) traversal to automatically
explore state combinations in the workflow persistence system
- Fix a real bug in `saveDraft()` discovered by the FSM test: orphan
cleanup in `loadIndex()` could delete a just-written payload when the
in-memory cache was empty

## Why this is needed

#9317 exposed a class of bug where two independently correct changes
interact to cause workflow loss. Conventional unit tests verify
specific, hand-picked scenarios and cannot catch these cross-PR
interaction bugs.

### AS IS (before)

| Aspect | Status |
|---|---|
| Testing approach | Example-based: developer picks specific inputs and
expected outputs |
| State coverage | Only explicitly written scenarios are tested |
| Cross-interaction bugs | Not detectable — each test runs one isolated
path |
| Bug in `saveDraft` | Undetected — `loadIndex()` orphan cleanup could
delete a just-written payload after `reset()` |

### TO BE (after)

| Aspect | Status |
|---|---|
| Testing approach | Property-based: fast-check generates **200 random
command sequences** per run |
| State coverage | Random exploration of `SaveDraft → GetDraft →
RemoveDraft → MoveDraft → GetMostRecentPath → Reset` combinations |
| Cross-interaction bugs | Detected automatically — fast-check shrinks
failing sequences to minimal reproductions |
| Bug in `saveDraft` | Found and fixed — `loadIndex()` now runs
**before** `writePayload()` to prevent orphan cleanup race |

## What fast-check does

fast-check is a property-based testing library. Instead of testing "does
this specific input produce this specific output?", it tests "does this
**property** hold for **all possible inputs**?"

For FSM testing specifically, fast-check:
1. Takes a set of **commands** (SaveDraft, GetDraft, RemoveDraft,
MoveDraft, GetMostRecentPath, Reset)
2. Generates **random sequences** of these commands
3. Runs each sequence against both a **model** (simplified oracle) and
the **real system** (store + localStorage)
4. Verifies **invariants** after every mutating command (index/payload
consistency, no orphans, LRU correctness, model agreement)
5. When a failure is found, **shrinks** the sequence to the minimal
reproduction

Example: the bug this PR fixes was shrunk to just 4 commands:
```
SaveDraft(d.json) → RemoveDraft(d.json) → Reset() → SaveDraft(a.json) ✗
```

## Changes

| File | Change |
|---|---|
| `package.json` / `pnpm-workspace.yaml` | Add `fast-check`
devDependency |
| `draftCacheV2.property.test.ts` | 7 property tests for pure index
functions |
| `workflowDraftStoreV2.fsm.test.ts` | FSM test: 6 commands, invariant
checking, 200 runs |
| `workflowDraftStoreV2.ts` | Fix: move `loadIndex()` before
`writePayload()` in `saveDraft()` |

## Test plan

- [x] `pnpm test:unit` — all 117 persistence tests pass (including 7
property + 1 FSM)
- [x] `pnpm typecheck` — clean
- [x] `pnpm lint` — clean
- [x] Pre-commit hooks pass (format, lint, typecheck)
- [x] Pre-push hook passes (knip)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9370-test-add-property-based-FSM-tests-for-workflow-persistence-3196d73d3650813daa98cdd8bef7e975)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: GitHub Action <action@github.com>

package.json

@@ -154,6 +154,7 @@
     "eslint-plugin-storybook": "catalog:",
     "eslint-plugin-unused-imports": "catalog:",
     "eslint-plugin-vue": "catalog:",
+    "fast-check": "catalog:",
     "fs-extra": "^11.2.0",
     "globals": "catalog:",
     "happy-dom": "catalog:",