feat: initial meshservice implementation

Generic decentralized service layer for mesh networks.
Includes FAPP (psychotherapy) and Housing as reference services.

- ServiceMessage with Ed25519 signatures
- Wire protocol (64-byte header + CBOR)
- ServiceRouter with pluggable handlers
- Verification framework (3 trust levels)
- 32 tests, 3 examples

target/debug/.fingerprint/syn-02256188bfb689a7/invoked.timestamp

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target/debug/.fingerprint/syn-02256188bfb689a7/lib-syn

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target/debug/.fingerprint/syn-02256188bfb689a7/lib-syn.json

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