fix(site-builder): swap build-stamp console.log for a window global, add test

Review found two real issues in the initial build-stamp commit:

- A permanent, unconditional console.log on every editor load for every
  customer is production noise. Replaced with a window.__WHP_EDITOR_BUILD__
  assignment -- same load-bearing effect (keeps build-stamp.ts from being
  tree-shaken out before Task 19 wires in the real call site), but prints
  nothing. Support can ask a user to type __WHP_EDITOR_BUILD__ in the
  console on request. Commented as load-bearing so it isn't later "cleaned
  up" as a stray global.
- editorBuild()'s 'dev' fallback was never actually exercised by any test
  in the suite, despite the previous report claiming otherwise. Added
  build-stamp.test.ts asserting editorBuild() === 'dev' under vitest.

Also silences the expected-failure stderr git prints on the successful
'nogit' fallback path (stdio: ['ignore', 'pipe', 'ignore']), so a
release-tarball build log doesn't show a misleading fatal: line for an
intentional, handled case.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

craft/vite.config.ts

@@ -9,7 +9,10 @@ import { execSync } from 'child_process'
 const editorBuild = (() => {
   let sha = 'nogit'
   try {
-    sha = execSync('git rev-parse --short HEAD', { cwd: __dirname }).toString().trim()
+    sha = execSync('git rev-parse --short HEAD', {
+      cwd: __dirname,
+      stdio: ['ignore', 'pipe', 'ignore'],
+    }).toString().trim()
   } catch {
     // Building outside a git checkout (release tarball) -- keep 'nogit'.
   }