feat(editor): Implemented / overridden gutter markers (#106)

Adds gutter markers for type hierarchies: ↓ "Is implemented / Has subclasses / Is overridden" on types and methods with implementations, ↑ "Implements / Overrides method in X" on methods that implement or override a supertype method. One target navigates, several open the Choose Implementation popup. Settings › Editor › General › Gutter Icons gets two toggles.

Markers come from the language server: a new batched `lsp_implementations` command (up to 4 concurrent, cancellable, sharing the goto query path) for ↓, and the Super Method lookup for ↑, which now prefers the overload with matching parameters (also fixes ⌘U on overloads). Only declarations near the viewport are queried, debounced, cached per document version and cancelled when superseded. Works for any server with implementation / type hierarchy support; servers without type hierarchy show ↓ only.

Fixes #99

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src-tauri/src/commands/lsp.rs

@@ -94,6 +94,25 @@ pub async fn lsp_goto(
     project!(window, state).lsp.goto(doc, pos, kind).await
 }
 
+/// Implementations of the symbols at `positions`, one list per position (the
+/// editor's "is implemented / overridden" gutter markers). Cancel with
+/// `cancel_request(request)`.
+#[tauri::command]
+#[specta::specta]
+pub async fn lsp_implementations(
+    window: Window,
+    state: State<'_, AppState>,
+    request: RequestId,
+    doc: DocId,
+    positions: Vec<Pos>,
+) -> IdeResult<Vec<Vec<Location>>> {
+    let lsp = project!(window, state).lsp.clone();
+    let cancel = state.begin_request(request);
+    let result = lsp.implementations(doc, &positions, cancel).await;
+    state.end_request(request);
+    result
+}
+
 /// Contents of a virtual document (library class, decompiled code) that a
 /// language server returned a non-`file:` URI for. Cached per project.
 #[tauri::command]

src-tauri/src/commands/mod.rs

@@ -144,6 +144,7 @@ pub fn builder() -> tauri_specta::Builder<tauri::Wry> {
         lsp::lsp_hover,
         lsp::lsp_signature_help,
         lsp::lsp_goto,
+        lsp::lsp_implementations,
         lsp::lsp_virtual_document,
         lsp::lsp_references,
         lsp::lsp_document_highlights,