test(java): unit tests for FR5d referrer/target population

Add Fr5dReferenceResolutionTest mirroring the TS reference in
server/typescript/packages/metadata/test/fr5d-reference-resolution-errors.test.ts.

Ten cases cover all five reference sites plus envelope-shape contracts:

  - extends: unresolved (single + cross-file)
  - @payloadRef unresolved → target=payloadRef string
  - @requiredSlots missing → target=`payloadRef.slot`
  - @via single-hop bad relationship → deepest-valid-prefix in message
  - @via multi-hop bad relationship → deepest-valid-prefix names the hops
    that DID resolve
  - @Of non-existent entity + @Of missing field on existing entity
  - resolved envelope carries referrer's files (so IDEs can jump to
    declaration site)
  - ResolvedSource.from() helper handles JsonSource and CodeSource fallback

Cross-port note in the class javadoc explains the Java vs TS FQN
divergence for top-level objects (Java propagates root `package:`, TS
does not).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing