refactor: scope certificate parser temporaries

[leopoldjoy]

Summary

• scope temporary ASN.1 pointers inside _parseTbs
• preserve existing parser checks and revert behavior
• reduce stack-depth pressure for the accepted parser-hardening fix stack

Context

This is an integration-only follow-up for the audit fix stack. It does not add a new validation rule; it only narrows local variable lifetimes so the parser-hardening PRs can combine without stack too deep.

Tests

• forge fmt --check src test script
• forge build --sizes src script
• forge test --match-path test/CertManager.t.sol

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