id(web): detectLanguage lets a special-name stem override a known extension — justfile.rs highlights as Just, makefile.md as Makefile #721

Summary

detectLanguage in pkgs/id/web/src/highlight.ts checks the file's stem against the special-filename table before it ever looks at the extension, so any file whose stem happens to be a special name is highlighted with the wrong language even when its extension unambiguously names another one.

Evidence (verified on main df38365, temporary vitest probe since removed)

highlight.ts:133-143 computes nameWithoutExt (basename minus its final extension) and returns FILENAME_TO_LANG[nameWithoutExt] before the extension lookup at highlight.ts:158-162.

Probe results against the real function:

Controls that must keep working:

Why it matters

The editor picks the Shiki grammar solely from this function (filename-detected language, highlight.ts createSyntaxHighlightPlugin). A mis-detected file is highlighted with a completely unrelated grammar — e.g. documentation about a Makefile (makefile.md) or a Rust source file named after the tool it drives (justfile.rs). The special-name rule was only ever meant for files that are the special file (Dockerfile, Makefile, CMakeLists.txt, Dockerfile.dev); it should not override a known extension.

Acceptance criteria

• When the basename's extension is present in EXT_TO_LANG, the extension wins over the stem special-name match (justfile.rs → rust, makefile.md → markdown, gemfile.json → json).
• Special filenames still win when there is no extension (Dockerfile, Gemfile, src/Makefile) or the extension is unknown (Dockerfile.dev, CMakeLists.txt).
• Dotfile handling (.env, .gitignore, .env.local, …) is unchanged.
• New regression tests in highlight.test.ts cover at least the stem-vs-known-extension cases and the unknown-extension controls above; the existing detectLanguage suite passes unchanged.