feat(syntax): detect languages from shebangs

[fcoury-oai]

Extensionless scripts currently have no automatic syntax selection even when their first line names an interpreter. This change detects languages from shebangs after existing filename and extension associations, so a file named script beginning with #!/usr/bin/env -S bash -eu gets Bash highlighting and syntax-aware editing.

Adds configurable languages.<id>.shebangs, bounded first-line detection, common env forms and numeric interpreter versions. Editor buffers, eligible previews, and diff sides use the same detector; edits and language reloads refresh the selected syntax. Manual syntax selection retains precedence. Zsh continues to use the bundled Bash grammar. LSP routing remains filename-based; quoted commands and arbitrary wrappers are outside this parser's scope.

Python pack interpreter metadata is a separate follow-up in codersauce/red-language-packs; this PR provides the detector and configuration support it needs.

How to Test

1. Create an extensionless file containing #!/usr/bin/env -S bash -eu followed by if true; then echo hello; fi. Open it with this build: Bash syntax should be highlighted without setting executable permissions. Scroll below the first line and confirm highlighting persists.
2. Change the shebang to an unknown interpreter: automatic highlighting should disappear. Use :syntax fish and then :syntax off to verify manual selection and disabling. A recognized .rs filename must still select Rust even with a Bash shebang.
3. Run cargo test --lib shebang: all five tests should pass, covering detection, registered interpreters, live reload, offscreen edits, manual selection, and preview cache invalidation.

Validated locally: the five shebang tests, 70 highlighter tests (cargo test --lib highlighter::tests), two preview tests (cargo test --lib preview_syntax_highlighting), formatting, and cargo clippy --all-targets --all-features -- -D warnings. The interactive steps above are reviewer instructions, not a claimed manual smoke test.

[fcoury]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-05T02:53:58.958171Z	6f1a133	Manual request

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