Prepare n2k v1 foundation

[jakthom]

Summary

• enforce conditional PGN fields, signed-width representability, ranges, and sentinel semantics in the schema codec
• prioritize mandatory protocol transmissions, surface their failures, and restore NMEA network citizenship after reconnect
• add owned raw observation access with source/network identity, capture metadata, replay support, and bounded delivery
• build the CLI on Cobra with sniff, record, replay, validate, devices, PGN lookup, structured help, typo suggestions, generated shell completion, and concrete typed physical output
• update the README demo with real messages from the bundled capture and add a just build recipe for bin/n2k
• add requirement-indexed local conformance evidence, deeper status metrics, architecture documentation, and regression coverage
• bump VERSION to 1.0.0

Verification

• go test ./...
• just pgn-sync-check
• just lint
• just secure
• just test-race
• just fuzz-smoke
• just conformance-local
• go test -race ./cmd/n2k
• just build
• xmllint --noout .github/demo.svg

All commands passed on the committed tree. The README SVG was also rasterized and visually checked at full width.

Release and conformance notes

The release workflow will create the v1.0.0 tag after merge and a successful main-branch run. Formal licensed review, hardware-in-the-loop validation, and the official NMEA certification workflow remain external work and are documented as not yet completed.