fix: reject ambiguous cert signature wrappers

[leopoldjoy]

Summary

• require the certificate signature BIT STRING to contain exactly one ASN.1 SEQUENCE
• require that sequence to contain exactly two INTEGER children for r and s
• reject substituted signature-wrapper tags and trailing ignored signature fields

Security value

This closes an outside-TBS certificate malleability surface where alternate signature wrappers could be accepted and interpreted differently from strict DER/X.509 parsers.

Tests

• forge fmt --check src test
• forge test --match-test 'test_VerifyCACertWithHints_RejectsSignatureWrapperTagSubstitution|test_VerifyCACertWithHints_RejectsTrailingSignatureFields|test_VerifyCACertWithHints_ShortS_Regression' -vvv

Note

Latest main already fails test_DeployableContractsFitEIP170 locally with CertManager runtime bytes 24,708 > 24,576, so full-suite verification remains blocked by that pre-existing size issue.