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/server/lib/git/routes.js

@@ -428,6 +428,49 @@ export function registerGitRoutes(app) {
     }
   });
 
+  app.get('/api/git/branch-base', async (req, res) => {
+    const { getBranchBase } = await getGitLibraries();
+    try {
+      const directory = resolveDirectoryQuery(req.query.directory);
+      if (!directory) {
+        return res.status(400).json({ error: 'directory parameter is required' });
+      }
+
+      const branch = resolveDirectoryQuery(req.query.branch);
+      if (!branch) {
+        return res.status(400).json({ error: 'branch parameter is required' });
+      }
+
+      const result = await getBranchBase(directory, branch);
+      res.json(result);
+    } catch (error) {
+      console.error('Failed to get branch base:', error);
+      res.status(500).json({ error: error.message || 'Failed to get branch base' });
+    }
+  });
+
+  app.get('/api/git/range-files', async (req, res) => {
+    const { getRangeFiles } = await getGitLibraries();
+    try {
+      const directory = resolveDirectoryQuery(req.query.directory);
+      if (!directory) {
+        return res.status(400).json({ error: 'directory parameter is required' });
+      }
+
+      const base = resolveDirectoryQuery(req.query.base);
+      const head = resolveDirectoryQuery(req.query.head);
+      if (!base || !head) {
+        return res.status(400).json({ error: 'base and head parameters are required' });
+      }
+
+      const files = await getRangeFiles(directory, { base, head });
+      res.json({ files });
+    } catch (error) {
+      console.error('Failed to get git range files:', error);
+      res.status(500).json({ error: error.message || 'Failed to get git range files' });
+    }
+  });
+
   app.post('/api/git/revert', async (req, res) => {
     const { revertFile } = await getGitLibraries();
     try {