Build a capability fixture corpus

[adrienlacombe]

Refs #135 (WF-B2). Builds on the provenance envelope shipped in #155.

Summary

• Adds a packaged corpus of canonical input fixtures for the seven WorldForge provider capabilities (predict, reason, embed, generate, transfer, score, policy). Each capability ships exactly one valid_baseline.json plus at least two invalid_<reason> fixtures with distinct expected_error_pattern entries — 21 fixtures total.
• Each fixture is a small JSON envelope (schema_version: 1) carrying an id, capability, data_class (synthetic | captured | host-supplied), expected outcome, expected error pattern, description, and a payload whose keys map directly onto the matching assert_*_conformance() keyword arguments — fixtures can be passed straight through without intermediate translation.
• The corpus lives under src/worldforge/testing/fixtures/ so it ships with the wheel and is reachable through the public testing API.
• Wires the corpus into tests/test_provider_contracts.py (mock-provider conformance for predict, reason, embed, generate) and adds 29 dedicated tests in tests/test_capability_fixtures.py that exercise every fixture: valid baselines run through the framework facade; invalid fixtures the framework already rejects trip WorldForgeError with their declared pattern; fixtures whose contract claim is not yet enforced assert their structural invariants and document the pin for future validators.
• Documentation: new public page docs/src/fixtures.md, contributor charter at src/worldforge/testing/fixtures/README.md, mkdocs nav and SUMMARY entries, CHANGELOG, and the WF-B2 acceptance checklist in docs/src/roadmap-continuation.md ticked.

Acceptance criteria (from #135)

• Each capability has at least one valid fixture and two invalid boundary fixtures.
• Fixtures are used by conformance or evaluation tests (tests/test_provider_contracts.py, tests/test_capability_fixtures.py).
• Fixture docs state whether data is synthetic, captured, or host-supplied.
• Package contract remains small and installable (no pyproject.toml change required; fixtures ship via the existing src/worldforge/testing/ package layout).

Public testing surface

New helpers re-exported from worldforge.testing:

Symbol	Purpose
CAPABILITY_FIXTURE_NAMES	Tuple of capability names covered by the corpus.
CapabilityFixture	Frozen dataclass returned by the loader.
FIXTURE_SCHEMA_VERSION	Currently 1; bump only with a loader migration.
list_fixture_names(capability)	Sorted file stems for a capability.
load_capability_fixture(capability, name)	Validate and return one fixture.
iter_capability_fixtures(capability)	Iterate validated fixtures for one capability.
iter_all_fixtures()	Iterate every fixture across the corpus.

Test plan

• uv run ruff check src tests examples scripts clean
• uv run ruff format --check src tests examples scripts clean
• uv lock --check
• uv run python scripts/generate_provider_docs.py --check
• uv run pytest — 679 passed / 2 skipped
• uv run --extra harness pytest --cov=src/worldforge --cov-report=term-missing --cov-fail-under=90 — 90.16 percent
• bash scripts/test_package.sh — fixtures ship in the wheel automatically
• uv run mkdocs build --strict

Migration

Pure addition. No public API was renamed or removed. Existing tests, conformance helpers, and provider fixtures under tests/fixtures/providers/ are untouched. Downstream issues (WF-B5 evidence bundles, WF-B7 failure-case galleries) can now consume worldforge.testing.iter_all_fixtures() instead of re-deriving inputs.