feat: add diffBranchesStat and diffFileSingle to BranchManager

Adds FileStatEntry type and two new primitives to the BranchManager
port and SimpleGitBranchManager adapter, enabling split diff
procedures in the tRPC layer without returning raw multi-megabyte diffs.

- FileStatEntry captures path, status, additions/deletions, oldPath
  (renames), and optional projectId for multi-project routing
- diffBranchesStat uses --name-status + --numstat, detects binary
  files (shown as - / - in numstat), handles spaces in filenames
- diffFileSingle returns raw unified diff for a single file path

apps/server/execution/orchestrator.test.ts

@@ -46,6 +46,8 @@ function createMocks() {
     ensureBranch: vi.fn(),
     mergeBranch: vi.fn().mockResolvedValue({ success: true, message: 'merged', previousRef: 'abc000' }),
     diffBranches: vi.fn().mockResolvedValue(''),
+    diffBranchesStat: vi.fn().mockResolvedValue([]),
+    diffFileSingle: vi.fn().mockResolvedValue(''),
     deleteBranch: vi.fn(),
     branchExists: vi.fn().mockResolvedValue(true),
     remoteBranchExists: vi.fn().mockResolvedValue(false),