tests: add fuzz targets for trace-frame encode/decode

[TeoSlayer]

Summary

• ReadFrame / WriteFrame / ReadEvent / WriteEvent are already fuzzed in zz_fuzz_frames_test.go. The trace-frame inner encoder/decoder (WriteTraceFrame + ReadTracePayload) was not.
• Adds two new fuzz targets in zz_fuzz_trace_test.go:
  • FuzzReadTracePayload — direct decoder fuzz with 5 seeds, including the one-byte-under-minimum guard case.
  • FuzzWriteThenReadTrace — round-trip property: encode arbitrary (innerType, sentAt, payload), then ReadFrame + ReadTracePayload, assert the three fields survive.

Local fuzz run (30s per target)

FuzzReadTracePayload     ~3.0M execs, +1 interesting corpus entry
FuzzWriteThenReadTrace   ~3.0M execs, +2

# Existing targets also re-ran clean (30s each):
FuzzDataExchangeReadFrame    ~170k execs, +7
FuzzDataExchangeRoundTrip    ~2.7M execs, +4
FuzzDataExchangeFileFrame    ~250k execs, +2
FuzzEventStreamReadEvent     ~2.4M execs, +6
FuzzEventStreamRoundTrip     ~3.3M execs, +1

Zero panics, zero crash-corpus entries. (One transient "context deadline exceeded" on FuzzDataExchangeFileFrame during back-to-back runs — did not reproduce on rerun, no testdata artefact written, attributed to CPU contention from running five fuzz targets sequentially. Not a real find.)

Test plan

• go test -race -count=1 -timeout 60s ./... — green (seed corpus runs as ordinary unit test)
• go test -run='^$' -fuzz=FuzzReadTracePayload -fuzztime=30s . — PASS
• go test -run='^$' -fuzz=FuzzWriteThenReadTrace -fuzztime=30s . — PASS
• git status clean (no testdata/fuzz/ artefacts)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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