feat: preview filing outcomes before activation

[tcballard]

What changed

• add a no-mutation preview from first-run setup and the rules editor for up to eight representative files
• show each file's status, exact matched rule, proposed filename, rendered destination, tags, controlled values, and decision reason
• reuse the shipping analyzer, compiled resolver, collision naming, extension checks, and validator while deliberately skipping Organizer.apply
• detect invalid templates, duplicate or conflicting routes, unreachable rules, and catch-all ordering mistakes before Save can activate them
• preserve draft and persisted rules until the person explicitly saves
• document the preview contract and regenerate the checked-in Xcode project

Product impact

People can now test the filing plan against real files before trusting it. Safe results are inspectable, unresolved values stay visibly in review, and structurally invalid rules cannot be activated.

The preview is intentionally evidence rather than a promise: a small sample helps explain the rules, but does not claim future model accuracy.

Validation

• swift test — 90 tests passed
• swift build --product SortingHatApp — passed
• no-signing Xcode build including the app, Finder extension, and Sparkle — passed
• XcodeGen reproducibility check — passed
• isolated real Foundation Models preview — receipt resolved to Receipts/Acorn Books/2026; both sample files remained unchanged and the output directory was never created
• Computer Use visual/AX pass — native table selection, actionable conflict state, long-value truncation/help, combined VoiceOver summaries

Closes #41