add stdin support, unit tests, and security audit ci

[thegdsks]

Summary

• 40 unit tests for parser, grade scoring, CertTime, and expiry display
• stdin support for inspect and decode commands (sslx inspect -, sslx decode -)
• 11 new integration tests (live TLS, JWT decode, cert/key mismatch, PEM roundtrip, stdin piping)
• cargo-audit security check added to CI pipeline

Test plan

• cargo test passes (101 tests)
• cargo clippy -- -D warnings clean
• cargo fmt --check clean
• CI passes on all 3 platforms

[Copilot]

Pull request overview

Adds stdin support and expands test coverage for sslx, plus introduces a dependency security audit job in CI.

Changes:

• Support reading certificate/JWT input from stdin for inspect and decode via -.
• Add unit tests for certificate parsing, grading boundaries, expiry display, and time conversion.
• Add new integration tests and a CI cargo-audit-based security audit job.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/integration.rs	Adds integration tests including stdin piping and live-host scenarios.
src/output/mod.rs	Adds unit tests for expiry_display formatting/color behavior.
src/main.rs	Updates CLI help text to document stdin usage (-).
src/commands/inspect.rs	Implements stdin support for inspect and factors rendering into run_certs.
src/commands/grade.rs	Exposes score_to_grade for testing and adds boundary tests.
src/commands/decode.rs	Implements stdin support for decode and reuses parsed certs via run_certs.
src/cert/parser.rs	Adds parse_cert_data and substantial parser unit test coverage.
src/cert/mod.rs	Exposes days_to_ymd for tests and adds CertTime/date conversion tests.
.github/workflows/ci.yml	Adds a new security audit job to CI.

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