fix: tolerate UTF-8 BOM in manifest; sync docs to app/app.old model

#1 BOM: PowerShell Set-Content -Encoding UTF8 writes a BOM that made the in-app
json.loads silently fail (banner never showed even with an update available).
- update_service: decode manifest with utf-8-sig (HTTP + local)
- build.ps1: write manifest.json without BOM (UTF8Encoding $false)
- tests: +2 covering BOM manifests (HTTP + local)

#2 docs: docs/10 §16 status synced to the implemented app/app.old/version.txt +
SHA-256 model (stages 2/3 done + e2e verified); tasks 17.18 updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_update_service.py

@@ -98,6 +98,14 @@ class TestCheckForUpdate(unittest.TestCase):
         self._write_manifest({"notes": "no version here"})
         self.assertIsNone(check_for_update(str(self.tmp), "1.0.0"))
 
+    def test_local_manifest_with_utf8_bom(self):
+        # PowerShell Set-Content -Encoding UTF8 emits a BOM; must still parse.
+        with open(str(self.tmp / "manifest.json"), "w", encoding="utf-8-sig") as f:
+            json.dump({"version": "1.1.0"}, f)
+        info = check_for_update(str(self.tmp), "1.0.0")
+        self.assertIsInstance(info, UpdateInfo)
+        self.assertEqual(info.version, "1.1.0")
+
     def test_source_falls_back_to_update_source(self):
         self._write_manifest({"version": "2.0.0"})   # no "source" field
         info = check_for_update(str(self.tmp), "1.0.0")
@@ -168,6 +176,15 @@ class TestCheckForUpdate(unittest.TestCase):
         with patch("services.update_service.request.urlopen", fake_urlopen):
             self.assertIsNone(check_for_update("https://example.test", "1.0.0"))
 
+    def test_http_manifest_with_utf8_bom(self):
+        def fake_urlopen(req, timeout=0):
+            return _FakeResponse('{"version": "2.0.0"}', raw=True)
+
+        with patch("services.update_service.request.urlopen", fake_urlopen):
+            info = check_for_update("https://example.test", "1.0.0")
+        self.assertIsInstance(info, UpdateInfo)
+        self.assertEqual(info.version, "2.0.0")
+
 
 class _FakeResponse:
     def __init__(self, data, raw=False):