fix(#2163): round-2 review fixes — deterministic fail-soft injection tests

- readGitStatus calls execFileSync via the child_process namespace so tests
  can inject spawn failures through the shared module object
- two deterministic tests: ERR_CHILD_PROCESS_STDOUT_MAXBUFFER-shaped and
  ETIMEDOUT-shaped throws both degrade to null (segment absent), proving
  the fail-soft paths the PR previously only asserted

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

hooks/gsd-statusline.js

@@ -6,7 +6,9 @@
 const fs = require('fs');
 const path = require('path');
 const os = require('os');
-const { execFileSync } = require('child_process');
+// Namespace (not destructured) so tests can inject spawn failures by
+// monkeypatching childProcess.execFileSync.
+const childProcess = require('child_process');
 const { isSemverNewer } = require('../gsd-core/bin/lib/semver-compare.cjs');
 const { PACKAGE_NAME, updateCacheFileName } = require('../gsd-core/bin/lib/package-identity.cjs');
 
@@ -339,7 +341,7 @@ function readGitStatus(dir) {
     // 8 MiB maxBuffer (default 1 MiB) headroom for repos with very many changed
     // or untracked files; overflow still degrades safely to segment-absent via
     // the catch below.
-    return execFileSync('git', ['-C', dir, 'status', '--porcelain=v2', '--branch'],
+    return childProcess.execFileSync('git', ['-C', dir, 'status', '--porcelain=v2', '--branch'],
       { encoding: 'utf8', timeout: GIT_STATUS_TIMEOUT_MS, maxBuffer: 8 * 1024 * 1024, stdio: ['ignore', 'pipe', 'ignore'] });
   } catch (e) {
     return null;