add SHA-based artifact-freshness check (#527)

[PolyphonyRequiem]

Summary

Adds VerbOutputSchemas_ArtifactIsFresh to VerbCatalogSanityTests — a SHA-based CI gate that fails when artifacts/verb-output-schemas.json drifts from what the current verb annotations would generate.

⚠️ RED UNTIL #527 Mozart-half merges

This test will fail until Mozart's [VerbResult] backfill PR (squad/527-verbresult-attrs) lands and regenerates artifacts/verb-output-schemas.json. That is expected and documented in the test's XML doc comment. Do not merge this PR to main until Mozart's half is also merged.

What this does

• In-process comparison: reads VerbOutputSchemaCatalog.Json (the compile-time constant produced by the VerbSchemaGenerator Roslyn source generator) and compares it against the on-disk artifacts/verb-output-schemas.json.
• Canonicalization before hashing: both sides are parsed and re-serialized with sorted object keys and consistent indentation before SHA-256 hashing. This prevents whitespace-only reformats from triggering false positives.
• Actionable failure message: reports both SHA values and tells the developer exactly what to run:

  Run: dotnet build src/Polyphony.SchemaExporter -c Release
  

Why SHA-based and not byte-for-byte?

The existing Catalog_ArtifactFile_ExistsAtRepoRoot_AndMatchesEmbeddedJson test already does a byte-for-byte comparison. This new test adds a normalized (canonical JSON) comparison layer on top, so that:

1. Formatting-only diffs don't create confusing "stale artifact" failures.
2. The failure message is unambiguous — SHA mismatch means semantic drift, not a trailing-newline difference.

Files changed

• tests/Polyphony.Tests/Annotations/VerbCatalogSanityTests.cs — new test + CanonicalizeJson/SortNode/Sha256Hex helpers

Part of #527 (Brahms half)

Mozart's half: squad/527-verbresult-attrs (backfill [VerbResult] on ~50 command methods and regenerate the artifact).