Make decoder schemas loadable and versioned

[zrack]

Operator outcome

A contributor can add a trusted decoder schema and fixtures without editing the core frame-processing code, and an operator can identify exactly which schema interpreted a session and evidence bundle.

Scope

• Move the canonical NSL-01 schema into a loadable, versioned registry.
• Define signature, trust, compatibility, and migration policies.
• Drive field decoding from schema definitions, including types, scales, enums, bounds, and units.
• Preserve imported schema bytes and cryptographic identity in sessions and .nlb bundles.
• Surface compatibility and unsupported-schema diagnostics without discarding raw records.

Invariants

• Raw source records remain immutable.
• Malformed and partial frames remain inspectable.
• Bundled and imported sessions continue through one validation/decode/replay/export pipeline.
• Existing NSL-01 v1 sessions remain reproducible.

Exit criteria

• A third-party schema plus fixtures can be added without changing core frame-processing code.
• Trust and compatibility failures have deterministic diagnostics.
• Conformance tests cover schema identity, versioning, decoding, migration, and evidence export.

[zrack]

Delivered through #27 and #28: NSL-01 and NMEA 0183 use the same bounded decoder registry and execution path; packs carry canonical content identity, schema/runtime compatibility, fixtures, deterministic diagnostics, session persistence, and evidence-bundle reproduction without pack-supplied JavaScript. Legacy NSL-01 sessions remain supported and unchanged.