Rewrite component/ to match A2UI v0.9 schema (1/2)

[joestump]

Summary

Rewrites the component/ package against the actual A2UI v0.9 basic catalog. Addresses REVIEW.md issue 2a — the initial scaffold's "kind"-discriminated, invented-catalog shape would never route a real A2UI document.

Spec verified against:

• https://a2ui.org/specification/v0.9-a2ui/
• https://a2ui.org/reference/components/

Wire shape (v0.9, flat-string): discriminator is "component" (not "kind"), values are PascalCase, children are referenced by ID via "child" (single) or "children" (array). Components are delivered as a flat adjacency list keyed by ID; component.Unmarshal decodes a single entry — the renderer assembles the tree.

Catalog implemented

Matches the 9 types in the brief, drawn from the v0.9 basic catalog:

Type	Notes
Card	child (single ID)
Button	child (single ID), variant, action (raw JSON, open shape)
Text	text is a Value (literal / path / call binding), variant
Row	children (ID array or template), justify, align
Column	same shape as Row
TextField	label, value (Value), textFieldType, validationRegexp
Modal	entryPointChild + contentChild
Tabs	tabItems[] of {title, child}
List	children (ID array or template), direction, align

All nine support accessibility (raw JSON) and weight.

Notable types

• Value — distinguishes literal ("John Doe"), path binding ({"path": "/contact/firstName"}), and function-call binding ({"call": "...", "args": {...}}); preserves raw JSON for lossless round-trips.
• ChildrenSpec — accepts both static ["id1", "id2"] and template object {"path": "/employees", "componentId": "employee_card_template"}.
• Action / Accessibility — kept as json.RawMessage (open shapes — let renderers/event routers decide).
• Per-type MarshalJSON re-emits the "component" discriminator so json.Marshal(card) produces a spec-shaped object.

Errors

No more silent _ = json.Unmarshal(...) swallowing. Unmarshal returns:

• ErrMissingDiscriminator when "component" is absent
• ErrUnknownComponent (wraps the offending value) for unknown types
• A typed fmt.Errorf wrapping the inner decode error for malformed payloads (e.g. Row.children: 42 → "decode Row: …")

Tests

component/component_test.go — table-driven, snippets sourced from the A2UI docs (URLs cited inline):

• One test per catalog entry, including both literal and path-binding Text, both static-array and template-object List
• Unknown component type → ErrUnknownComponent
• Missing discriminator → ErrMissingDiscriminator
• Malformed JSON (empty, trailing garbage, unclosed object, discriminator not a string)
• Malformed payload that passes the discriminator check (e.g. Row.children: 42)
• Nested round-trip: a Card → Column → two Buttons → two Text labels decoded from a 6-entry adjacency list, asserting IDs line up
• Kind-constant smoke test pinning each Kind* constant to the spec's PascalCase value

go test ./component/... passes; go vet ./component/... clean; gofmt -l component/ examples/ clean.

sample.json

examples/hello/sample.json is now a real v0.9 updateComponents envelope (Card → Column with greeting Text, subtitle Text, and a Row of OK/Cancel Buttons with Text labels). 11 lines of components inside a 13-line file.

KNOWN: this PR alone breaks go build ./...

This is expected and called out by the brief. render/* (and a small amount of a2tea.go) still reference the old component names. Specifically, go build ./... from main of this branch fails with:

render/choice.go:14:14: undefined: component.Choice
render/form.go:14:14: undefined: component.Form
render/input.go:14:14: undefined: component.Input
render/markdown.go:14:14: undefined: component.Markdown
render/progress.go:14:14: undefined: component.Progress
render/stream.go:14:14: undefined: component.Stream

This is Agent 2's job in PR #2 (stacked on fix/a2ui-schema). PR #2 will rebuild render/* against the new catalog, fix the a2tea.Render entry-point (REVIEW.md issue 2c), wire huh/v2 (REVIEW.md issue 2b), and restore a green full-tree build.

Scope kept clean: this PR only touches component/component.go, component/component_test.go, and examples/hello/sample.json.

Test plan

• go test ./component/... passes
• go vet ./component/... clean
• gofmt -l component/ examples/ clean
• cd component && go build . succeeds
• Confirmed go build ./... fails only on the expected render/* references (Agent 2)

Do not merge before PR #2 lands — the tree won't build.

Refs REVIEW.md issue 2a.