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>

scripts/build.ps1

@@ -158,7 +158,13 @@ $Manifest = [ordered]@{
     min_supported = $MinSupported
     notes = $ReleaseNotes
 }
-$Manifest | ConvertTo-Json | Set-Content -LiteralPath $ManifestPath -Encoding UTF8
+# Write UTF-8 WITHOUT BOM: Set-Content -Encoding UTF8 emits a BOM that breaks
+# json.loads in the in-app update check (services/update_service.py).
+[System.IO.File]::WriteAllText(
+    $ManifestPath,
+    ($Manifest | ConvertTo-Json),
+    (New-Object System.Text.UTF8Encoding $false)
+)
 
 if ($PublishDir) {
     New-Item -ItemType Directory -Force -Path $PublishDir | Out-Null