feat(traceability): preserve verification result provenance

[LunaMeerkats]

This adds a narrow traceability query that preserves where verification results came from. It is a foundation for #2232, without choosing result aggregation or presentation semantics yet.

The new VerificationResultProvenance record keeps the requirement actually verified, an optional generated test case, and the raw result node. TraceabilityIndex.get_verification_result_provenance() follows the two graph shapes StrictDoc already creates:

• IS_SATISFIED_BY for a direct requirement-to-result link;
• IS_VERIFIED_BY followed by HAS_RESULT for a generated test-case link.

The query also traverses ordinary requirement hierarchy in deterministic order. This lets a consumer distinguish a result attached to the selected requirement from one attached to a descendant. It deliberately does not normalize producer-specific statuses, mutate a requirement's own STATUS, aggregate pass/fail state, or add HTML indicators.

The integration fixture builds the real SDoc, C++, and CTest/JUnit graph. It covers direct and mediated links, a two-level requirement hierarchy, exact ordering, duplicate prevention, raw failed-result preservation, and unchanged requirement statuses.

Validation:

• invoke test-integration --focus verification_result_provenance --no-parallelization
• invoke test-unit --path tests/unit/strictdoc/core/test_traceability_index.py
• targeted Ruff format and lint checks
• invoke lint-mypy
• invoke lint-commit

[LunaMeerkats]

Main has moved through 0.28.1 since this opened, and GitHub now reports conflicts in traceability_index.py and the release notes. I checked current main: no equivalent provenance query has landed, and the graph APIs used by this change are still present. Resolving the conflicts requires a one-commit rebase that combines the import changes and moves the note into the current Unreleased section.

I do not rewrite published branch history. I can open a replacement PR from current main; a maintainer would then need to close this one. Please confirm if you want me to take that route.