Enforce source-reference and provider-mapping contracts #70

Depends on #68 and #69. Add repository validation and focused tests for the asset-class page inventory, MkDocs navigation, wrapped-provider names, provider-page targets, and required comparison-table fields against the provider registry. Tests must demonstrate failures for an unknown provider class, a missing navigation entry, and an incomplete source row. Keep live URL verification separate from the deterministic offline lane.

Update the provider index, provider-selection guide, and related provider pages so they link to authoritative asset-class references without repeating inventories. Review the complete diff for unsupported claims, duplicate descriptions, stale names, and accidental runtime changes.

Verification:

• Focused validator tests pass and fail for each specified invalid fixture.
• The validator passes against the final documentation and provider registry.
• Changed external links pass live verification or carry recorded evidence of an automated-access restriction.
• Ruff, formatting, ty, both pytest lanes, strict MkDocs, package build, actionlint, and pre-commit all pass.

Done means every specification criterion passes and the public diff contains no provider runtime changes.

Plan reference: ml4t-data-dev/.workspace/work/asset-class-source-reference/plan.md - Issue 3 in milestone 0.2.1 - Asset-class data-source reference.