fix(render,chat): A2UI datamodel back-propagation (Finding K)

[blove]

Summary

A2UI input components (TextField, MultipleChoice, CheckBox, Slider, DateTimeInput) have always emitted `a2ui:datamodel::` strings via their `emit` callback when the user changes value. No consumer existed for that format — neither libs/render's `emitFn` nor libs/chat's `` handled the magic-string protocol — so user input went nowhere. Forms rendered but were inert: typing in a TextField didn't update `surface.dataModel`, validation never saw input, Submit stayed disabled, binding-dependent UI never updated.

This was tracked as Finding K through Phase 4 and Phase 5 of the canonical demo's evolution.

Two coordinated fixes

1. `libs/render/src/lib/render-element.component.ts` — `emitFn` interception

`emitFn` now checks for the `a2ui:datamodel:` prefix before delegating to the `el.on[event]` handler binding. When matched, it parses out the path (split on the LAST colon — paths may contain colons per RFC 6901, and values certainly can: URLs, time strings, JSON arrays), coerces the raw string value back to a typed form, and writes via `this.ctx.store.set(path, value)`. The store triggers re-render with the new resolved value.

Type coercion handles: numeric literals (`/^-?\d+(?:\.\d+)?$/`), boolean literals (`true`/`false`), JSON-stringified arrays (MultipleChoice emits these), else string passthrough.

2. `libs/chat/src/lib/a2ui/surface-to-spec.ts` — preserve path refs as `$bindState` markers

Path-bound props were eagerly resolved at conversion time (`resolveDynamic` returned the literal value), so catalog components received a static snapshot that never changed even after datamodel writes hit the store. Fix: leave path refs as `{$bindState: '/path'}` markers — json-render's prop resolution reads the live state store on every render, so user writes propagate immediately. Literal DynamicValues (`{literalString: ...}`, `{literalNumber: ...}`, etc.) still resolve eagerly.

The `_bindings` map (prop name → path) is preserved so catalog components emit the magic string with the correct path on user input.

Verification

Live smoke against the canonical demo's feedback form:

• TextField receives focus; `(input)` event fires on user typing
• Catalog `onInput` calls `emitBinding` with prop name + new value
• `emitBinding` emits `a2ui:datamodel:/form/name:Brian`
• render-element's `emitFn` intercepts → `store.set('/form/name', 'Brian')`
• json-render's prop resolution sees the new state on next render
• TextField's `text()` input resolves to "Brian" via `$bindState`
• UI updates reactively with the typed value

Confirmed via direct JS introspection:
```js
ng.getComponent(document.querySelector('a2ui-text-field'))?.text?.() // "Brian"
```

Two surface-to-spec tests updated to assert the new `$bindState` marker behavior instead of the old eager-resolution snapshot.

Out of scope (deferred)

• Syncing json-render store back into `A2uiSurfaceStore.surface.dataModel` — for cross-render persistence (thread reload, view in chat-debug). For Phase 5 the in-form interactivity is the user-visible win; the round-trip (Submit → action message → re-submit) already captures values from the live store via `buildA2uiActionMessage`.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
cacheplane	Ready	Preview, Comment	May 10, 2026 4:30am