test: LLVM coverage, libfuzzer targets, and ConPTY

[fxrdhan]

Summary

Introducing:

1. Automated Line & Branch Code Coverage (LLVM-Cov): Dedicated CI job using cargo-llvm-cov to generate LCOV reports and step summaries for workspace tests and doctests.
2. Coverage-Guided Fuzzing Framework (fuzz/ Crate): Continuous LLVM libfuzzer-sys fuzz targets with AddressSanitizer (ASan) for Tar archives, YAML themes, LS_COLORS, and duration parsers, accompanied by a fast CI smoke canary.
3. Windows Native Virtual Console (ConPTY) Testing: Win32 console mode invariants and Virtual Terminal Processing testing (ENABLE_VIRTUAL_TERMINAL_PROCESSING) in tests/platform/windows_conpty.rs.
4. Linux Syscall Count Invariant Guard (strace): Automated verification script devtools/verify-syscall-invariants.sh guarding against $O(N^2)$ metadata syscall amplification on FUSE/NFS workloads.
5. Adversarial Test Optimizations: Streamlined sibling lookup fixtures to guarantee sub-second execution while maintaining full invariant depth.

Type of Change

• 🐛 Bug fix (non-breaking change fixing an issue)
• ✨ New feature (non-breaking change adding functionality)
• ⚡ Performance improvement
• ♻️ Code refactor / clean-up
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation / Man pages / Completions
• 🔧 Build / CI / Dependencies

Related Issues & Upstream References

• Resolves: Test suite expansion & gold standard coverage
• Upstream: N/A

How Has This Been Tested?

• Testing commands executed:
  • cargo nextest run --workspace (2,294 tests passed)
  • cargo test --doc (29 doctests passed)
  • cargo clippy --all-targets --all-features -- -D warnings (passed cleanly)
  • cargo fmt --all -- --check (passed)
  • reuse lint & convco pre-commit hooks (passed)
• Platforms verified:
  • macOS (Apple Silicon Darwin arm64)
  • Linux & Windows CI target matrices

Contributor Checklist

• My commits follow Conventional Commits format
• Upstream references are included in this PR body only, not in commit titles
• Tests covering the changes have been added/updated
• cargo clippy --all-targets passes with no warnings
• cargo nextest run (or cargo test) passes
• cargo fmt --check / nix fmt passes
• License headers (SPDX / REUSE) are properly preserved/added