keep-frost-net: split KfpMessage::validate into per-variant validators

[kwsantiago]

KfpMessage::validate was a 552-line match carrying the wire-validation checks (size caps, index bounds, charset/prefix checks, derivation-path rules, xpub parsing, etc.) for every message variant inline. This splits each variant's checks into its own validator.

Changes

• Each non-trivial arm's body becomes a private free function validate_<variant>(p: &<Variant>Payload) -> Result<(), &'static str> (28 of them).
• validate (552 → 33 lines) is now a dispatch match returning each validator's result, with _ => Ok(()) for the variants that carry no checks (SignatureComplete, DuressBeacon, Ping, Pong).

Every resulting function is under 100 lines (largest: validate_psbt_propose at 80).

Behavior

Pure code motion. Every check, threshold constant, error string, and helper call (decode_pcr_values, is_valid_fingerprint, Xpub::parse, the nonce-ref/sentinel logic, etc.) is unchanged. The _ => Ok(()) catch-all preserves the original _ => {} + trailing Ok(()) behavior for the four variants that were already validation-free; RefreshRound1 and RefreshRound2, which DO carry checks, are dispatched explicitly (not folded into the catch-all).

Verification

• cargo clippy -p keep-frost-net --all-targets: clean.
• keep-frost-net unit suite: 434 passed.
• multinode integration suite (--features testing --test multinode_test): 40 passed.

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Review details

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between b382711 and ba1f58b.

📒 Files selected for processing (1)

• keep-frost-net/src/protocol.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/frost-net-validate-split

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