Raptorcast: fix commitment

[xinyuan-dev]

This PR implements the validation of secp256k1 signature and restrict deterministic raptorcast commitment to a subset of headers that's relevant to encoding scheme, excluding the signature itself.

[Copilot]

🟡 Changes recommended

A few changes are needed to avoid avoidable panics and to correct/clarify guarantees (notably signature “unique byte representation” wording and use of constants in tests).

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Pull request overview

This PR tightens signature canonicalization by rejecting high-S secp256k1 signatures at deserialization time, and adjusts deterministic raptorcast commitment logic to commit only to a deterministic subset of chunk header fields (including a coarse timestamp bucket) relevant to the encoding scheme.

Changes:

• Enforce low-S secp256k1 signatures during SecpSignature::deserialize, updating tests accordingly.
• Make raptorcast commitments deterministic by committing to (round, global_merkle_root, encoding_scheme_variant, app_message_len, coarse_ts) rather than signature bytes.
• Introduce EncodingScheme::variant() and a shared timestamp bucketing helper (TIMESTAMP_BUCKET_MS / coarse_ts) for deterministic encoding inputs.

File summaries

File	Description
monad-secp/src/secp.rs	Reject high-S signatures at deserialization; update malleability test to fail at decode time.
monad-raptorcast/src/util.rs	Add EncodingScheme::variant() helper to expose the header variant byte.
monad-raptorcast/src/round_info.rs	Redefine commitment claim to a deterministic header subset and expand commitment conflict tests.
monad-raptorcast/src/packet/deterministic.rs	Add timestamp bucketing constant/function and use it in seed derivation.

Review details

Suppressed comments (1)

monad-raptorcast/src/round_info.rs:540

• This test hard-codes the next timestamp bucket boundary (2048). Prefer referencing deterministic::TIMESTAMP_BUCKET_MS so the test remains correct if the bucket size changes.

            .try_commit(&dummy_chunk(10, &MERKLE_A, LEN_A, 2048))

• Files reviewed: 4/4 changed files
• Comments generated: 3
• Review effort level: Lite

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