[Epic] Unify the client interactivity & communication model

[cssbruno]

Architectural plan to reduce the client interactivity surface from ~10 concepts to ~5 orthogonal primitives, fix the determinism integrity risk at its root, and turn the bounded→WASM cliff into a ladder.

Today's surface (~10)

state, computed, effect, props, scoped slots, emit, exports, g:bind, page stores, app-global stores, instance/context state, realtime.

Target (~5 primitives)

1. Reactive state with a scope axis — state @island|@page|@app|@instance, one lifecycle. → [Architecture] Unify reactive state under one primitive with a scope axis (state @scope) #517
2. computed / effect — unchanged.
3. Communication = callback props (actions) + scoped cells (state); bind: is sugar. → [Architecture] Collapse parent-child communication into callback props + scoped cells #518
4. Scoped slots — unchanged.
5. No event bus — [Components] Native inter-island event/message channel (page-scoped + app-global pub/sub) #514 (closed).

Workstreams

• [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 — single-source client semantics (one IR + one evaluator; generate validator AND runtime). Foundational — gates all client-language expansion.
• [Architecture] Unify reactive state under one primitive with a scope axis (state @scope) #517 — unify reactive state under state @scope (supersedes [Components] App-global / cross-package stores with ownership + teardown contract #508, [Components] Instance-scoped state for compound/headless components (context's DX, islands-native) — needs design #515).
• [Architecture] Collapse parent-child communication into callback props + scoped cells #518 — collapse parent-child into callback props + scoped cells.
• [Architecture] Callable pure Go helpers from the bounded client (WASM-cliff middle rung) #519 — pure Go helpers from the bounded client (WASM-cliff middle rung).

Gated on #384 (don't widen the divergence surface first)

#501 (iteration), #504 (switch/match).

Orthogonal (unaffected by this epic)

#502, #503, #505, #506, #507, #509, #510, #511, #512.

Considered & rejected

Event bus (#514, closed); full-page hydration; resumability (Qwik-style).

Principles

Islands not hydration · bounded / Go-owned / deterministic · explicit ownership over implicit coupling.