conformance: per-fixture versioning model

[jithinraj]

Summary

The current conformance fixture layout uses a single manifest.json that lists all fixture files with a global format_version. As the number of conformance categories grows (interaction evidence, policy binding, RFC 9421 proof, etc.), a per-fixture versioning model would improve maintainability.

Current state

• specs/conformance/fixtures/manifest.json has a top-level format_version and a flat fixtures[] array
• All fixture files live in specs/conformance/fixtures/ with category subdirectories (e.g., interaction/)
• New categories (like interaction/rfc9421-proof.json) are added by appending to the manifest

Proposed change

Move from "category-wide single version" to "fixture-file versioned":

• Each fixture file includes its own schema_version or format_version field
• The manifest becomes an index (paths + descriptions) without owning the version
• Tooling validates each fixture against the version it declares

Why

• Independent evolution: a new proof-capture fixture shouldn't require bumping the global version
• Clearer compatibility: consumers know exactly which fixture format they're validating against
• Aligns with how extension schemas already version independently (rfc9421-proof/0.1/schema.json)

Context

Filed from PR #379 (RFC 9421 proof capture profile) review feedback.