Context

bmalloc/libpas causes crashes on Windows under memory pressure during GC. See #28097, #26985, #26982.

The fix requires a coordinated change between oven-sh/WebKit and oven-sh/bun:

Upstream (oven-sh/WebKit)

Re-add -DUSE_SYSTEM_MALLOC=ON to the cmake invocation in windows-release.ps1:

 cmake -S . -B $WebKitBuild `
     -DPORT="JSCOnly" `
     -DENABLE_STATIC_JSC=ON `
+    -DUSE_SYSTEM_MALLOC=ON `
     ...

This was previously added in WebKit PR #97 (July 2025) but was removed in a later update. The current prebuilt WebKit for Windows (commit 00e825523d) does not have this flag.

Downstream (oven-sh/bun) — after prebuilts are rebuilt

1. Remove bmalloc.lib from Windows link targets in cmake/targets/BuildBun.cmake
2. Exclude bmallocLib(cfg) on Windows in scripts/build/deps/webkit.ts (TODO comments already added in fix(windows): disable bmalloc/libpas linking to fix crash #28098)
3. Add CI regression guard (assertNoBmallocOnWindows)

Background

• WebKit PR Installs for some dependencies are deferred to the end when they should be eager #97: Originally added -DUSE_SYSTEM_MALLOC=ON for Windows
• Bun PR webkit: disable libpas on windows #20931: Removed bmalloc.lib from link targets (worked with the WebKit fix)
• Bun PR Update WebKit #26381: WebKit update — new version dropped the flag, re-enabling bmalloc in prebuilts