test(conformance): add golden artifact corpus

[szmyty]

Closes #366

Summary

• add a versioned CC0 synthetic corpus spanning document, paged document, raster/vector image, audio, video, archive, structured data, subtitle, corrupt, extension/magic mismatch, unknown binary, and ordered collection cases
• add a hermetic production-path harness covering deterministic planning/execution, shared intermediates, cross-family output, aggregation, cache behavior, provider fallback, validation and branch-local failures, cancellation, checkpoint resume/invalidation, hygiene gates, bounded extraction, bundled profiles, and Flow artifact projection
• add Draft 2020-12 schemas for the corpus and machine-readable conformance report, and map covered formats into the existing capability matrix
• disambiguate RIFF and ISO base media detection by form/major brand so valid WAVE and MP4 fixtures do not produce false conflicts
• add documented fast/tool-backed/maximal tiers plus scheduled and release report publication

Focused checks

• changed-file rustfmt --check
• cargo check --workspace --all-features --locked
• focused Clippy with warnings denied
• golden conformance harness in fast and maximal report modes
• focused detector tests (32 passed)
• focused capability-matrix test
• Draft 2020-12 validation of both the corpus and emitted report schemas
• conformance workflow YAML parse and git diff --check

The full test suite and GitHub Actions were intentionally deferred for this speed-run.

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