languages: Add built-in Kotlin support via kotlin-language-server

[jasonsmithio]

Summary

Adds Kotlin as a built-in language. .kt and .kts files get syntax highlighting, indent rules, outline, and completion / hover / go-to-def via kotlin-language-server — found on $PATH first, otherwise downloaded as server.zip from fwcd/kotlin-language-server GitHub releases. Same zip works on all platforms because the launcher is a shell script and the implementation is a JVM JAR.

Second of four planned bake-in PRs (Swift → Kotlin → Java → PHP). Independent of #28 — both base on main and don't conflict.

Grammar crate selection (the hard part)

Three published Kotlin tree-sitter crates, only one usable:

Crate	tree-sitter constraint	Resolves against workspace 0.26?	Node shape
tree-sitter-kotlin 0.3.8	<0.23	❌ links conflict	simple_identifier (matches Zed extension queries)
tree-sitter-kotlin-ng 1.1.0	^0.24	❌ minor-version conflict	identifier / qualified_identifier (would force rewriting every .scm)
tree-sitter-kotlin-codanna 0.3.9	>=0.21 (open-ended)	✅	simple_identifier (preserves original)

Went with codanna. Rationale is recorded in a multi-line comment on the ("kotlin", tree_sitter_kotlin_codanna::language()) entry in crates/grammars/src/grammars.rs so the next reader doesn't try to switch back to "the canonical" crate.

What's here

• Grammar. tree-sitter-kotlin-codanna = "0.3.9" (MIT). Added to workspace Cargo.toml, crates/grammars/Cargo.toml (deps + load-grammars feature), and registered in crates/grammars/src/grammars.rs::native_grammars().
• Query files. Pulled from zed-extensions/kotlin (MIT, compatible with the GPL-3.0 languages crate) into crates/grammars/src/kotlin/: brackets.scm, config.toml, highlights.scm (one patch), indents.scm, injections.scm, outline.scm (one patch), overrides.scm.
• LSP adapter. New crates/languages/src/kotlin.rs (~190 LOC). Modeled on c.rs::CLspAdapter (GitHub-zip pattern). check_if_user_installed does delegate.which("kotlin-language-server"); fetch_latest_server_version queries fwcd/kotlin-language-server for the single server.zip asset (constant name across releases); fetch_server_binary downloads, unzips into <container>/kotlin-language-server_<tag>/server/, returns server/bin/kotlin-language-server[.bat]. Sets 0o755 on the launcher post-unzip on Unix. Skips the --version validity probe used by c.rs because the launcher shells out to java — every probe would boot a JVM; file-existence check + digest comparison is enough.
• Wiring. crates/languages/src/lib.rs::init gains a mod kotlin;, an Arc::new(kotlin::KotlinLspAdapter) instantiation, and a LanguageInfo { name: "kotlin", adapters: vec![kotlin_lsp_adapter], .. } entry between jsonc and markdown. All four touch points tagged // PaddleBoard: for upstream-merge resolution.

Query fixups discovered during smoke testing

The zed-extensions/kotlin queries were authored against a slightly different grammar shape than codanna's. Two .scm files needed minor patches:

• highlights.scm line 139: anonymous "null" token → (null_literal) named node. Codanna grammar models null as a named node, the quoted form failed with Invalid node type "null".
• outline.scm lines 24-39: dropped the ["val" "var"] @context matchers inside property_declaration. Codanna's property_declaration only exposes named children to queries (no anonymous val/var tokens), so the original pattern was Impossible. Property names still appear in the outline, just without the val/var prefix.

Both patches have explanatory ; PaddleBoard: comments inline.

Test plan

• cargo check -p grammars --features load-grammars clean
• cargo check -p paddleboard clean
• Language registers cleanly in the running binary (zero query-loader errors after the .scm patches)
• Live LSP spawn — needs a .kt buffer to trigger the ~85 MB download. Mechanically identical to c.rs's clangd flow which is known to work; recommend the first reviewer verify by git checkout + open a .kt file with kotlin-language-server not on $PATH (force the download path).

Known limitations

• JDK requirement is runtime-detected, not pre-checked. The launcher script bails clearly if java isn't on $PATH. fwcd's recent releases want JDK 17+; older Java will surface a stderr error from the launcher rather than a friendly notification. Adding a java -version probe is a worthwhile follow-up.
• Property val/var no longer appears as outline context (see fixup above).
• No Gradle / buildSrc multi-module integration — Kotlin LSP needs path-style classpath hints for non-trivial projects. Out of scope here; users can set up settings.json overrides.

Out of scope (follow-ups)

• Java / PHP — next two PRs in the bake-in series
• JDK version pre-check + clearer error notification
• Gradle multi-module classpath integration
• Custom completion ranking for Kotlin

Release Notes:

• Added built-in support for Kotlin, including syntax highlighting and language-server features (completion, hover, go-to-definition) via kotlin-language-server from fwcd/kotlin-language-server. The server is downloaded automatically on first use, or picked up from $PATH if installed system-wide. Requires a JDK at runtime.