Add verification script for issue #27 (autosave batching)

[AchieverSana]

Summary

Investigated issue #27 (autosave creating many individual IndexedDB transactions per drawn object, causing slow saves). Traced the full object-commit path used by every drawing tool:

endStroke() / finishShape() (js/modules/tools.js) → commitIfChanged() (js/core/history.js) → debounced scheduleAutosave() (400ms, js/modules/persistence.js) → single dbSaveBoard() → one readwrite transaction via tx.put().

This confirmed the debounce and single-transaction batching described as the desired fix in #27 are already implemented and correctly wired for every drawing tool (pen, shapes, text, etc). No functional code change was needed.

Added a lightweight browser-console verification script (scripts/verify-autosave-batching.js) that simulates adding 100 objects rapidly and asserts the benchmark from #27: fewer than 10 IndexedDB transactions and completion well within the expected time budget.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / cleanup
• Other (describe below)

Verification / regression-test script — no application code was changed.

Test plan

• Tested in a modern browser (Chrome)
• Drawing, selection, undo/redo still work (unaffected — no app code touched)
• No new console errors on load
• Ran scripts/verify-autosave-batching.js in the browser console after drawing 100 simulated objects: transaction count and timing both passed the benchmark from [Performance]: Autosave creates many individual transactions; IndexedDB slow #27

Screenshots (if UI changed)

N/A — no UI changes.

Related issues

Closes #27