feat(diff): add branch scope to context panel diff view

Show every change on the current branch relative to its base in the
Changed/Staged/Last turn dropdown. The base comes from the branch's
reflog record or an explicit per-branch user choice (persisted), never
a main/master guess; when git has no record the user picks a base once
from a searchable branch list.

- server: GET /api/git/branch-base (reflog-derived base),
  GET /api/git/range-files (name-status -z with rename/copy
  destination paths and -C copy detection)
- shared UI: optional getBranchBase/getGitRangeFiles runtime APIs
  with boundary parsing; persisted per-branch overrides keyed by
  runtime+directory+branch
- DiffView: branch scope with confirmed-unavailability coercion of
  persisted tabs (detached HEAD, default-branch checkout, metadata
  settled without a default), range-invalidated diff cache guarded
  against stale completions, bounded branch-metadata retry, read-only
  diff actions in branch scope; hidden in VS Code
- helper module branchDiffScope.ts with tests for coercion,
  availability, race conditions, and retry exhaustion

packages/ui/src/lib/gitApiHttp.ts

@@ -3,6 +3,7 @@ import type {
   GitDiffResponse,
   GetGitDiffOptions,
   GetGitRangeDiffOptions,
+  GetGitRangeFilesOptions,
   GitFileDiffResponse,
   GetGitFileDiffOptions,
   GitBranch,
@@ -248,6 +249,51 @@ export async function getGitRangeDiff(
   return response.json();
 }
 
+export async function getGitRangeFiles(
+  directory: string,
+  options: GetGitRangeFilesOptions
+): Promise<import('./api/types').GitRangeFileEntry[]> {
+  const { base, head } = options;
+  if (!base || !head) {
+    throw new Error('base and head are required to fetch git range files');
+  }
+
+  const response = await runtimeFetch(
+    buildUrl(`${API_BASE}/range-files`, directory, { base, head })
+  );
+
+  if (!response.ok) {
+    throw new Error(`Failed to get git range files: ${response.statusText}`);
+  }
+
+  const payload = (await response.json()) as { files?: unknown };
+  if (!Array.isArray(payload.files)) return [];
+  return payload.files.filter((entry): entry is import('./api/types').GitRangeFileEntry => {
+    if (!entry || typeof entry !== 'object') return false;
+    const candidate = entry as { path?: unknown; status?: unknown };
+    return typeof candidate.path === 'string' && typeof candidate.status === 'string';
+  });
+}
+
+export async function getBranchBase(
+  directory: string,
+  branch: string
+): Promise<import('./api/types').GitBranchBaseResponse> {
+  if (!branch) {
+    throw new Error('branch is required to get branch base');
+  }
+
+  const response = await runtimeFetch(
+    buildUrl(`${API_BASE}/branch-base`, directory, { branch })
+  );
+
+  if (!response.ok) {
+    throw new Error(`Failed to get branch base: ${response.statusText}`);
+  }
+
+  return response.json();
+}
+
 export async function getGitFileDiff(directory: string, options: GetGitFileDiffOptions): Promise<GitFileDiffResponse> {
   const { path, staged } = options;
   if (!path) {