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/api/types.ts

@@ -157,6 +157,22 @@ export interface GetGitRangeDiffOptions {
   contextLines?: number;
 }
 
+export interface GetGitRangeFilesOptions {
+  base: string;
+  head: string;
+}
+
+/** One changed file in a `base...head` range, with its change letter (A/M/D/R/C). */
+export interface GitRangeFileEntry {
+  path: string;
+  status: string;
+}
+
+export interface GitBranchBaseResponse {
+  /** Null when git has no authoritative record of where the branch started. */
+  base: string | null;
+}
+
 export interface GitFileDiffResponse {
   original: string;
   modified: string;
@@ -466,6 +482,8 @@ export interface GitAPI {
   getGitDiff(directory: string, options: GetGitDiffOptions): Promise<GitDiffResponse>;
   getGitFileDiff(directory: string, options: GetGitFileDiffOptions): Promise<GitFileDiffResponse>;
   getGitRangeDiff?(directory: string, options: GetGitRangeDiffOptions): Promise<GitDiffResponse>;
+  getGitRangeFiles?(directory: string, options: GetGitRangeFilesOptions): Promise<GitRangeFileEntry[]>;
+  getBranchBase?(directory: string, branch: string): Promise<GitBranchBaseResponse>;
   revertGitFile(directory: string, filePath: string, options?: { scope?: 'all' | 'working' }): Promise<void>;
   stageGitFile(directory: string, filePath: string): Promise<void>;
   stageGitFiles?(directory: string, filePaths: string[]): Promise<void>;