fix dep and sec issues

[dhuseby]

multi-sig silently swallows malformed ThresholdData
multi-sig/src/views/bls12381.rs:554,645,688:

Ok(b) => ThresholdData::try_from(b).unwrap_or_default(),

If ThresholdData parsing fails (corruption, truncation, tampering), unwrap_or_default() returns an empty ThresholdData. The caller then sees zero shares and may proceed with an empty threshold set. This is a fail-open pattern — corrupted security-critical data is silently replaced with a safe-looking default rather than surfacing an error.

fix: Propagate the error with ? or return a ThresholdError::InvalidThresholdData.

multi-sig duplicate share identifiers silently overwrite
multi-sig/src/views/bls12381.rs:314 and multi-key/src/views/bls12381.rs (analogous) — shares.insert(identifier, sdata) silently overwrites an existing share with the same identifier. In threshold reconstruction, a duplicate identifier means one participant's share is lost, potentially dropping below the threshold without any error.

fix: Check contains_key before insert and return DuplicateShare error.

multi-sig BLS codec inferred from signature length
multi-sig/src/views/bls12381.rs:419-429 and multi-key/src/views/bls12381.rs:459-465 — The BLS curve (G1 vs G2) is selected by signature byte length (48 → G1, 96 → G2). This is a heuristic, not cryptographic binding. A 48-byte G2 signature or 96-byte G1 signature would be misclassified. If downstream code trusts the codec tag for curve selection, this could cause verification on the wrong curve.

fix: Require explicit codec specification rather than length-based inference, or document that this heuristic is a known limitation.

multi-sig inconsistent error handling between G1/G2 combine
multi-sig/src/views/bls12381.rs:685-688 (G2 path) uses av.payload_encoding().ok() (optional — silently proceeds if missing), while line 740 (G1 path) uses av.payload_encoding()? (required — returns error). This asymmetry means G2 combine silently accepts missing payload encoding while G1 rejects it.

multi-sig typo in error message
multi-sig/src/error.rs:74 — "Signature missing limi" should be "Signature missing limit".

Signed-off-by: Dave Grantham dave@cryptid.tech