chore(engine-ambient): audit script for static-write regressions

Step 9 of multi-instancing migration. PowerShell audit fails CI if anything
references the deleted BattleManagerBase.main field or introduces a new
Thread() outside the LeanThreadPool allowlist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tools/engine-port/audit-static-writes.ps1

@@ -0,0 +1,33 @@
+# Multi-instancing audit: fail if any per-battle static is written directly outside its own declaring file.
+$errors = @()
+
+# new Thread( allowlist — only LeanThreadPool's login-subsystem worker is OK.
+$threadAllowlist = @('SVSim.BattleEngine/Engine/Cute/LeanThreadPool.cs')
+$threadHits = Get-ChildItem -Recurse -Filter *.cs -Path SVSim.BattleEngine,SVSim.BattleNode |
+    Select-String -Pattern 'new\s+Thread\s*\(' |
+    Where-Object {
+        $rel = ($_.Path -replace '\\','/').Substring($PWD.Path.Length + 1)
+        $threadAllowlist -notcontains $rel
+    }
+if ($threadHits) {
+    $errors += "Unallowed new Thread( usage:`n" + ($threadHits -join "`n")
+}
+
+# Direct static assignments to BattleManagerBase.IsForecast / IsRandomDraw / main outside its file.
+$staticWrites = Get-ChildItem -Recurse -Filter *.cs -Path SVSim.BattleEngine/Engine,SVSim.BattleNode |
+    Where-Object { $_.Name -ne 'BattleManagerBase.cs' } |
+    Select-String -Pattern 'BattleManagerBase\.(main|IsForecast|IsRandomDraw)\s*='
+if ($staticWrites) {
+    # OK to set IsForecast/IsRandomDraw via property setter — those route through ambient now.
+    # But flag any 'BattleManagerBase.main =' as the deleted field.
+    $bad = $staticWrites | Where-Object { $_.Line -match 'BattleManagerBase\.main\s*=' }
+    if ($bad) {
+        $errors += "Reference to deleted BattleManagerBase.main:`n" + ($bad -join "`n")
+    }
+}
+
+if ($errors) {
+    Write-Error ($errors -join "`n`n")
+    exit 1
+}
+Write-Host "Audit clean."