test: with I/O error isolation, signal cleanup, and syntax fuzzing

[fxrdhan]

Summary

• I/O Error Isolation: Tests partial filesystem failure (EACCES / 000 permissions) across Grid, Long (-l), Tree (-T), JSON (--json), and LOC (--code) views, ensuring errors are isolated per-entry while preserving sibling results and valid output streams.
• Signal Handling & Process Cleanup: Adds SIGINT (Ctrl+C) and SIGTERM termination tests during active directory traversals, ensuring clean process exit and zero zombie processes.
• Tar Archive Corruption Fuzzing: Adds concatenated tar archives, truncated blocks in the middle of files, and trailing garbage resilience tests to archive_fuzz_stress.rs.
• LOC Engine Syntax Edge Cases: Adds parser validation for TypeScript template literals with interpolations (${...}), Ruby and Perl multi-line heredocs, and HTML/XML comment structures.

Type of Change

• ♻️ Code refactor / clean-up (Test suite expansion)
• 🔧 Build / CI / Dependencies

Related Issues

• Resolves: N/A

How Has This Been Tested?

• cargo fmt --all -- --check
• cargo clippy --all-targets --features git,inspect-archives -- -D warnings
• cargo nextest run --workspace (2,323 tests passing)
• cargo test --doc (29 doctests passing)
• cargo check across all 4 feature permutations (none, git, inspect-archives, all-features)

Contributor Checklist

• My commits follow Conventional Commits format
• 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