Modernize scanner architecture and tooling

[script3r]

Summary

• unify the CLI and library scanning pipeline, bound work queues, and avoid redundant library scans
• make optional language features safe, fix C-header fallback, output self-scanning, gitignore toggling, and thread validation
• upgrade core dependencies and migrate parsing to the Tree-sitter 0.26 progress callback API
• strengthen CI, package contents, MSRV metadata, documentation, and regression coverage

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --locked -- -D warnings
• cargo clippy --all-targets --no-default-features --locked -- -D warnings
• cargo test --all-features --locked
• cargo test --no-default-features --locked
• every individual language feature builds
• cargo +1.88.0 check --all-targets --all-features --locked
• cargo package --allow-dirty --locked

patterns.toml and fixture contents are unchanged.