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 Only declarations near the viewport are queried, debounced, cached per document version and cancelled when superseded.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/features/settings/pages/editor/general.tsx

@@ -536,6 +536,18 @@ const GUTTER = editorPage({
               component: () => <GutterGroup id="run" label="Run configurations and tests" />,
             },
             { t: "custom", label: "Bookmarks", component: () => <GutterGroup id="bookmarks" label="Bookmarks" /> },
+            {
+              t: "custom",
+              label: "Implemented and overridden",
+              keywords: ["implementations", "subclasses", "inheritors"],
+              component: () => <GutterGroup id="inheritors" label="Implemented and overridden declarations" />,
+            },
+            {
+              t: "custom",
+              label: "Implementing and overriding",
+              keywords: ["super method", "overrides", "implements"],
+              component: () => <GutterGroup id="supers" label="Implementing and overriding methods" />,
+            },
           ],
         },
       ],