verify: Kani harnesses for signature-section parser (#88)

[avrabe]

Summary

Advances #88. The varint and DSSE Kani harnesses already exist; the signature-section parser was the remaining gap.

Adds an inline #[cfg(kani)] mod proofs block to sig_sections.rs covering SignatureForHashes::deserialize — the point where raw untrusted signature bytes are structurally decoded and length-checked, before any Ed25519/SHA-256 crypto runs.

Four harnesses:

• proof_deserialize_short_input_no_panic — every 4-byte symbolic input returns Ok/Err, never panics.
• proof_deserialize_truncated_length_field_rejected — a declared key_id length that outruns the buffer is rejected, no OOB read.
• proof_deserialize_empty_input_rejected — empty input rejected.
• proof_deserialize_unsupported_alg_rejected — non-Ed25519 algorithm id rejected at parse time.

All target the parsing surface only — no crypto loops — so the only loops Kani unwinds are the 5-iteration LEB128 varint loops, covered by per-harness #[kani::unwind(6)] within --default-unwind 4.

Test plan

• cargo build -p wsc clean
• Harness logic reviewed against SignatureForHashes::deserialize — each is_err() assertion holds, no panic paths
• CI Kani sig_sections job — masked (tolerate_failure: true) on first landing since Kani could not run in the authoring env; flip to false once observed green

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[avrabe]

Closing — the sig_sections harness is not Kani-tractable as written (BufReader). See the analysis on #88; reopening the work there once deserialize is refactored to expose a BufRead-taking entry point.