feat(evals): add legend-state evals to async-state

[mikolajadamowicz]

Summary

Adds Legend State (@legendapp/state v3) as a new baseline library in the async-state category, with 5 focused evals covering its fine-grained observable model. Each eval ships a prompt, atomic requirements, a baseline app/App.tsx, and a reference implementation.

async-state grows from 13 → 18 evals; the suite total goes from 134 → 139.

Evals Added

#	Eval	Focus
15	rn-legend-state-observable-async-action-lifecycle	observable store, explicit idle/loading/success/error action lifecycle, tracked reads via useValue/use$/observer
16	rn-legend-state-computed-derived-state-batch	Computed functions inside the observable, batch for multi-field writes, Memo/Computed fine-grained rendering
17	rn-legend-state-persist-asyncstorage-hydration-gate	syncObservable + AsyncStorage persist plugin, hydration gating on syncState(...).isPersistLoaded
18	rn-legend-state-reactive-two-way-input	$TextInput two-way binding via $value, computed dirty flag, Show-driven save bar
19	rn-legend-state-async-observable-lazy-load	Async observable activating lazily on first tracked read, loading/error derived from syncState instead of useEffect + useState

Category Documentation

evals/async-state/README.md gains a Legend State baseline entry, v3 doc references (including the v2 → v3 migration notes for persistObservable → syncObservable and useSelector → useValue), and six new traceability rules:

• D19 — read observables through tracked contexts, not untracked .get() in render
• D20 — model derived values as computed functions in the observable
• D21 — batch multi-field writes to avoid intermediate renders and persistence churn
• D22 — gate persisted UI on isPersistLoaded
• D23 — two-way bind inputs with reactive $ components instead of `useState
• D24 — model lazily-loaded server data as async observables with syncState

Policies Enforced

• Leak policy — prompts describe the scenario and name the library, but never the graded target identifiers (batch, Memo, syncState, isPersistLoaded, $value, etc. are requirement-side only where applicable)
• Guard policy — no baseline-passable requirements; each baseline app/App.useState/useEffect` version the prompt asks to replace
• Deterministic data — network evals use the pinned dummyjson.com endpoint patterns and documented response shapes; evals 16 and 18 are fully local

Other Changes

• testbench/package.json: adds @legendapp/state@^3.0.0-beta.48
• README.md and paper/benchmark-methodology-whitepaper.tex: updated eval counts and the async-state category scope to include Legend State

[artus9033]

LGTM, thanks for the contribution!