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/web/src/api/git.ts

@@ -11,6 +11,8 @@ export const createWebGitAPI = (): GitAPI => ({
   getGitDiff: gitApiHttp.getGitDiff,
   getGitFileDiff: gitApiHttp.getGitFileDiff,
   getGitRangeDiff: gitApiHttp.getGitRangeDiff,
+  getGitRangeFiles: gitApiHttp.getGitRangeFiles,
+  getBranchBase: gitApiHttp.getBranchBase,
   revertGitFile: gitApiHttp.revertGitFile,
   stageGitFile: gitApiHttp.stageGitFile,
   stageGitFiles: gitApiHttp.stageGitFiles,