feat!: Legend state port.

[stephenh]

Migration Complete: MobX → Legend-State v3

Results: 167/167 tests passing, build clean, zero MobX references remaining.

What changed

Production code (7 files):

• rules.ts Removed action() wrapper — plain function
• utils.ts Removed isObservable/toJS; added isClassInstance for deepClone
• setupTests.ts Removed configure({ enforceActions })
• fields/fragmentField.ts observable({value:1}) → observablePrimitive(1), removed makeAutoObservable
• fields/valueField.ts All mutable state (touched, _readOnly, _loading, _focused, rules) backed by Legend-State observables. Early return when setting same value.
• Removed MobX class support.
• fields/listField.ts Tick pattern migrated. add/remove/set/commitChanges wrapped in batch(). Child value tracking via getter chain (no observe propagation).
• fields/objectField.ts Same pattern — tick + batch + getter-based child tracking. set/revertChanges/commitChanges batched. No makeAutoObservable.

Key architectural decisions:

1. No makeAutoObservable replacement — getters work through Legend-State dependency tracking via .get() calls on observablePrimitive ticks
2. Getter-based child propagation instead of observe() reactions — avoids infinite loops with observer HOC
3. Explicit batch() for multi-mutation operations — MobX auto-batched, Legend-State doesn't
4. Early return on same-value set — avoids unnecessary observer notifications

Tests (6 files):

• autorun → observe (1-arg)
• reaction → observe (2-arg, with e.num > 0 guard to skip first fire)
• Observer render prop → observer HOC wrapping
• Removed: MobX class tests, observable array wrapping test, mobx-behavior.test.ts

Known limitation

The observer HOC from @legendapp/state/react causes infinite re-render loops when used with useFormState (the form state hook modifies observables in useEffect that observer tracks). The FormStateApp demo component works without observer because useFormState already handles re-rendering. This needs investigation — likely a Legend-State v3 beta interaction with React's effect lifecycle.