docs(spec): add golden test corpus reference (§7.5) (#4)

Add a new subsection to §7 Wire Examples referencing the golden
test corpus at testdata/wire/ in the implementation repository
as the authoritative byte-level wire format reference.

Includes links to the corpus directory, the TestWireFormatGolden
invariance gate test, and regeneration instructions.

Closes PILOT-422

Author: matthew-pilot

SPEC.md

@@ -408,6 +408,22 @@ Byte  8-19: [12-byte nonce]
 Byte 20+:   [ciphertext + 16-byte GCM tag]
 ```
 
+### 7.5 Golden Test Corpus
+
+The wire examples above are illustrative. The authoritative byte-level wire format reference is the **golden test corpus** maintained at [`testdata/wire/`](https://github.com/TeoSlayer/pilotprotocol/tree/main/testdata/wire) in the implementation repository.
+
+This corpus contains byte-for-byte snapshots of every Pilot Protocol wire frame across layers L1, L4, L5, L6, and L7, including packet headers, relay messages, key exchange frames, encrypted payloads, and SACK acknowledgements.
+
+Each `.bin` file is parsed by the live production decoders, re-marshalled, and asserted byte-identical by [`TestWireFormatGolden`](https://github.com/TeoSlayer/pilotprotocol/blob/main/tests/zz_wire_golden_test.go). Any wire-format regression — a field reorder, an extra padding byte, a checksum tweak — fails the test and names the offending frame.
+
+**Regenerating the corpus** is a wire-format change and requires explicit version-bump approval. The generator lives at `cmd/wire-gen/main.go` behind the `wirecorpus` build tag:
+
+```
+go run -tags=wirecorpus ./cmd/wire-gen
+```
+
+See the [`testdata/wire/README.md`](https://github.com/TeoSlayer/pilotprotocol/blob/main/testdata/wire/README.md) for the full frame inventory and regeneration instructions.
+
 ---
 
 ## 8. Version Negotiation