test(DEN-4080): normalize authored fixtures for TJSV corpus

[ORESoftware]

Summary

Preserve the production repository's independently authored fixture bytes while deriving a declaration-indexed corpus for the pinned ORESoftware/typespec-json-schema-validator differential gate.

TJSV intentionally treats the first corpus directory as the declaration (instances/<Declaration>/...). ORES Chat production fixtures are currently arranged as <slice>/<Declaration>.json, so passing that tree directly makes admin, conversations, etc. look like declaration names. This PR fixes the acceptance action rather than weakening or disabling TJSV's corpus checks.

Behavior

• snapshot TypeSpec A, independently authored JSON Schema A, and the original fixture tree before derivation;
• read declaration names from JSON Schema $defs rather than hard-coding ORES Chat slices;
• preserve already-canonical TJSV layouts (<Declaration>/valid|invalid/*.json and <Declaration>/*.json);
• route production <slice>/<Declaration>.json to an isolated evidence-only <Declaration>/<slice>--<Declaration>.json copy;
• retain an explicit reviewed mapping for legacy ticket-created.json -> SupportEvent;
• reject unknown declarations, unsafe nesting, symlinks, non-regular files, invalid JSON, and output collisions fail-closed;
• verify byte-for-byte SHA-256 equality between each authored fixture and its derived copy;
• make the composite action pass only the derived corpus to pinned TJSV 24e3bb322b8d7c9cd2e0c7dd185d659ad3fd7328 while the immutability receipt continues to bind the original authored inputs.

Tests

• unit tests cover canonical valid/invalid routing, production slice routing, legacy routing, unknown declarations and unsafe nesting;
• CI derives a production-style support/Widget.json layout from the existing convergent fixture and executes the complete composite action against it;
• existing convergent, deliberate-drift, numeric-positive, and wrong-int64-encoding negative canaries remain required.

No authored TypeSpec, JSON Schema, or fixture source becomes generated authority. Normalized corpus files are downstream evidence only.

Linear: DEN-4080; related DEN-3828 and DEN-3959.