Define the Dart a2ui_agent API and its tests, limited to protocol v0.9 - #2408

Notifications bypassed the signal's equality check (`force: true`), so every
observer on a path related to a write was woken, including observers whose own
value was unchanged. Watching /a/b and then writing /a fired the observer even
though /a/b was absent before and after.

The check was bypassed because a Map or List mutated in place keeps its
identity and would compare equal. Hand the signal a copy of a container
instead: containers still compare unequal and still notify, while unchanged
primitive and absent values no longer do. This is what the web_core renderer
already does.

Verified: an unchanged descendant no longer fires; a root replacement still
wakes the root observer but not an unrelated one; an ancestor of an in-place
mutation still fires; rewriting a path with the value it already holds does not.

The two implementations now agree, so the behaviour moves into the shared
suite: core/data_model.yaml regains the notification assertion on root
replacement and gains cases for an unchanged descendant and a same-value
rewrite. The corresponding exclusion note and the web_core local test are
removed.

…-and-tests' into dart-a2ui-agent-api-and-tests

…-and-tests' into dart-a2ui-agent-api-and-tests

…-and-tests' into dart-a2ui-agent-api-and-tests

Comment #4 on PR 2408 asked to remove a2ui_core changes; the suite was
dropped because v1_0 already carries 51 cases under the same filename
with an incompatible case shape. Restored instead as the two cases v1_0
does not cover -- component and data model isolation between surfaces --
written in the v1_0 vocabulary so the files concatenate on merge.

Also fixes two suite headers that cited the wrong blueprint section:
catalog providers are 3.F and transformers 3.A, not 3.B and 3.C.

Prerequisite for the Dart a2ui_agent API (#2408). Everything here is a
change to a2ui_core, or to a consumer of it, split out so the agent PR
reviews as agent work only.

Catalog and capabilities
- `Catalog<C extends ComponentApi, F extends FunctionApi>` (breaking,
  0.1.1 -> 0.2.0). Agents parameterise with `CatalogFunction` (signature
  only), renderers with `FunctionImplementation`. `SchemaCatalog` aliases
  the agent shape.
- `Catalog.fromJson` / `catalogSchema` / `copyWith`, plus schema-only
  `CatalogComponent` and `CatalogFunction`, so catalog documents round
  trip through core and a narrowed catalog renders a narrowed document
  with `$defs/anyComponent` and `$defs/anyFunction` narrowed to match.
- `A2uiRendererCapabilities`, mirroring `client_capabilities.json` and
  web_core's `A2uiClientCapabilities`.
- `A2uiProtocolVersion`, and the `A2uiParseError` / `A2uiCompileError` /
  `A2uiCatalogError` / `A2uiIntegrityError` / `A2uiRecursionError`
  categories.
- `A2uiValidator`, with the v0.9 version gate implemented and the
  structural and catalog-schema checks declared but stubbed.

One bug fixed
- `DataModel.set` silently dropped a write whose parent path resolved to
  a primitive (`/user/name/first` where `/user/name` is a string). It now
  throws `A2uiDataError`, matching web_core. The shared dataset surfaced
  this. Notification on an unchanged value is fixed the same way: the
  signal is handed a copy of a container rather than bypassing the
  equality check.

Shared conformance data
- `core/data_model.yaml` (new, `data_model` action): 37 cases migrated
  from `renderers/web_core/src/v0_9/state/data-model.test.ts`.
- `core/message_processor.yaml` (new, `process_messages` action): the two
  surface-isolation cases the v1_0 branch's suite of the same name does
  not cover, written in that branch's case vocabulary so the two files
  concatenate rather than conflict when it lands.
- `conformance_schema.json` gains the two actions and the `DataError`
  category; the file's existing formatting is preserved.

web_core consumes the shared data
- `tests/conformance/harness.ts` locates `conformance/` by walking up.
- `data-model.conformance.test.ts` and
  `message-processor.conformance.test.ts` run the two suites. Both are
  additive: the hand-written `data-model.test.ts` and
  `message-processor.test.ts` are untouched.

Also
- `blueprints/modules/a2ui_core.blueprint.md`: package boundary and
  non-goals, catalog immutability and per-catalog protocol version, the
  version-keyed capabilities objects and their normative schemas, the
  complete exception hierarchy plus the rule that parsing wire JSON
  raises from it, and a conformance section (there was none).
- `flutter_packages_test.yml` discovers packages under `dart/` as well as
  `samples/`. The `dart/` packages were not built, analyzed or tested by
  any workflow before this.
- `dart/a2ui_agent` gets the two changes that exposure requires: its
  `a2ui_core` constraint follows the major bump, and the unnecessary
  `library;` directive its stub carried is removed.

'a capabilities payload above all' said nothing precise; it is just an
example, so name it as one. Also applies prettier, which the previous
commit skipped.

Follows 562329b on dart-a2ui-core. A renderer can need a smaller
catalog for a given use case and derives one the same way. What the
agent SDK owns is the named transformer rules and the CatalogConfig
pipeline, which is the actual reason their cases live under agent/.

The per-item prose restated what the a2ui_agent blueprint already
documents. A list of what is not in core, with one pointer to where it
is specified, carries the same boundary without the duplication.