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/gitApi.ts

@@ -119,6 +119,24 @@ export async function getGitRangeDiff(
   return gitHttp.getGitRangeDiff(directory, options);
 }
 
+export async function getGitRangeFiles(
+  directory: string,
+  options: import('./api/types').GetGitRangeFilesOptions
+): Promise<import('./api/types').GitRangeFileEntry[]> {
+  const runtime = getRuntimeGit();
+  if (runtime?.getGitRangeFiles) return runtime.getGitRangeFiles(directory, options);
+  return gitHttp.getGitRangeFiles(directory, options);
+}
+
+export async function getBranchBase(
+  directory: string,
+  branch: string
+): Promise<import('./api/types').GitBranchBaseResponse> {
+  const runtime = getRuntimeGit();
+  if (runtime?.getBranchBase) return runtime.getBranchBase(directory, branch);
+  return gitHttp.getBranchBase(directory, branch);
+}
+
 export async function revertGitFile(
   directory: string,
   filePath: string,