docs(architecture): client interactivity & communication model plan (#520)

[cssbruno]

Addresses the epic #520 (which stays open as a tracker — an epic closes when its workstreams land).

#520 is an [Epic]: an "architectural plan to reduce the client interactivity surface from ~10 concepts to ~5 orthogonal primitives." This PR delivers that plan as docs/engineering/client-interactivity-model-plan.md, tying the workstreams and their decisions together.

What the plan captures

• Today's surface (~10) and where the redundancy concentrates (state sharing expressed 3+ ways; parent↔child expressed 3 ways that already share one transport).
• Target (~5 orthogonal primitives): scoped reactive state @scope; computed/effect; communication = callback props + scoped cells (bind: as sugar); scoped slots; no event bus.
• The [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 gate: every client-language expansion is gated on single-source semantics (one IR + one evaluator generating both validator and runtime). The RuntimeExpressionSpec parity pattern is the seed; [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 generalizes it. Widening the surface before [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 multiplies Go/JS divergence.
• Workstream sequencing: [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 → [Architecture] Unify reactive state under one primitive with a scope axis (state @scope) #517 (scoped state) → [Architecture] Collapse parent-child communication into callback props + scoped cells #518 (callback props/scoped cells) / [Architecture] Callable pure Go helpers from the bounded client (WASM-cliff middle rung) #519 (Go helpers), with [Client] Iteration and slice/struct return types in computed and helper functions #501/[Client] switch/match expressions and multi-statement helper functions #504 explicitly gated behind [Client] Client expression DSL has two independent, diverging implementations (Go vs hand-written JS) #384 but outside the primitive set.
• Migration posture (v0.6.0 lane-rename precedent: precise nudges, no silent aliases) and considered & rejected (event bus [Components] Native inter-island event/message channel (page-scoped + app-global pub/sub) #514, full-page hydration, resumability).

Relationship to the sibling PRs

This epic plan references the decisions made in:

• ADR 0016 ([Architecture] Callable pure Go helpers from the bounded client (WASM-cliff middle rung) #519, PR docs(architecture): decide pure Go helpers from the bounded client (ADR 0016, #519) #616) — pure Go helpers compile to WASM from Go source.
• ADR 0017 ([Architecture] Collapse parent-child communication into callback props + scoped cells #518, PR docs(architecture): callback props + scoped cells for parent-child (ADR 0017, #518) #617) — callback props + scoped cells; deprecate emit/exports/g:bind.

Those ADR files live on their respective branches; the cross-references resolve once those PRs merge. References here are plain text (no broken markdown links).

Scope

Documentation only — the epic's architectural plan. No code changes; the workstreams implement under their own issues.