astro(feat[parity]): Add Symbol schema export and Pydantic↔Zod parity

why: Cycle 19 closes Step 4 (autodoc → typed Symbol collection).
Symbols now have the same dual-schema contract as the doctree:
Pydantic emits canonical JSON Schema, the TS theme has hand-written
Zod that validates the same shape, behavioural parity tests prove
identical fixture payloads pass through both paths via
z.fromJSONSchema. Symbol.docstring_body transitively pulls every
doctree node model into the Symbol schema, so this single schema
file covers both the API-symbol shape AND the doctree subset that
docstrings render through.

what:
- Add export_symbol_schema() to schemas.py returning
  Symbol.model_json_schema() (transitively includes Parameter,
  SymbolSource, and every BlockNode/InlineNode under $defs)
- Re-export from gp_sphinx_astro_builder.__init__
- AstroBuilder.finish() also writes <outdir>/schemas/symbol.schema.json
  alongside doctree.schema.json
- Commit astro/fixtures/symbol.schema.json (1137 lines, Pydantic
  canonical export)
- Commit astro/fixtures/hello-symbol.json — canonical Symbol
  fixture for parity testing (function with two parameters,
  populated docstring_body and source)
- Extend test_schema_fixture.py: split the single drift-detection
  test into doctree + symbol variants
- Add 4 schemas.py pytest tests (returns dict, defs membership for
  Parameter+SymbolSource, canonical Symbol round-trip, full schema
  syrupy snapshot)
- Add astro/packages/theme/src/schemas/symbol.ts with hand-written
  Zod schemas (parameterKindSchema, parameterSchema,
  symbolSourceSchema, symbolKindSchema, symbolSchema). Symbol
  imports blockNodeSchema from doctree.ts so docstring_body inherits
  the doctree validation.
- Add 10 symbol.test.ts validation tests
- Add 8 symbol-parity.test.ts behavioural-parity tests:
    * fixture loading (defs + top-level shape)
    * canonical hello-symbol passes hand-written symbolSchema
    * canonical hello-symbol passes z.fromJSONSchema(pydantic)
    * malformed kind rejected by both paths
    * missing required field rejected by both paths
    * test.each over the 2 symbol-specific $defs

Verification: Python 1370 passed (was 1364) / 3 skipped, 15
snapshots green; TS 88 vitest tests pass (was 70), type-check
clean, biome lint+format clean. Step 4 of notes/plans/astro.md is
complete: autodoc directives now produce typed Symbol entries in
src/content/api/symbols.json with Pydantic↔Zod parity end-to-end.

Author: tony

astro/fixtures/hello-symbol.json

@@ -0,0 +1,40 @@
+{
+  "id": "demo_api.merge_demo",
+  "kind": "function",
+  "name": "merge_demo",
+  "qualname": "merge_demo",
+  "module": "demo_api",
+  "signature": "(project: str, version: str = \"0.0.0\") -> dict[str, str]",
+  "parameters": [
+    {
+      "name": "project",
+      "annotation": "str",
+      "default": null,
+      "kind": "positional"
+    },
+    {
+      "name": "version",
+      "annotation": "str",
+      "default": "\"0.0.0\"",
+      "kind": "positional"
+    }
+  ],
+  "returns": "dict[str, str]",
+  "docstring_summary": "Merge a tiny pseudo-config payload.",
+  "docstring_body": [
+    {
+      "type": "paragraph",
+      "children": [
+        {
+          "type": "text",
+          "value": "Merge a tiny pseudo-config payload."
+        }
+      ]
+    }
+  ],
+  "source": {
+    "repo": "https://github.com/git-pull/gp-sphinx",
+    "path": "demo_api.py",
+    "line": 4
+  }
+}