feat(parser): add CSS support

[rabii-chaarani]

Summary

• register CSS discovery for .css
• add the Tree-sitter CSS grammar, named-node syntax catalog, CLI, and MCP exposure
• preserve complete ordered stylesheet syntax without inventing selector ontology mappings
• document the conservative parsing contract in k-wiki

Verification

• cargo fmt --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --locked
• cargo publish --dry-run --locked
• exact crate-size gate
• recommended wiki validation, link check, and site build