test: stub Range layout methods once in the test setup (#53)

Root cause: jsdom has no layout and CodeMirror measures text ranges in an animation frame, sometimes after a test finished; test files that did not stub Range.getClientRects (e.g. search/codePreview) threw an unhandled TypeError that failed whole CI runs at random (seen on #41).
Fix: stub Range.getClientRects / getBoundingClientRect once in src/test/setup.ts; drop the six per-file copies.
Verified: vitest on editor/lsp, editor/refactor, templates, search (55 files, 458 tests, no unhandled errors).

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

src/editor/lsp/syntaxFallback.test.ts

@@ -35,9 +35,6 @@ const { startRename } = await import("./rename");
 const { lspDoc } = await import("./lsp");
 const { useLspPopup, closePopup } = await import("./popupStore");
 
-Range.prototype.getClientRects ??= () => [] as unknown as DOMRectList;
-Range.prototype.getBoundingClientRect ??= () => new DOMRect();
-
 const r = (line: number, col: number, endCol: number): Range => ({ start: { line, col }, end: { line, col: endCol } });
 const node = (kind: string, range: Range, field: string | null = null): SyntaxNode => ({
   kind,