fix: disable cppgc caged heap for HermitOS

On x64, cppgc_enable_caged_heap defaults to true, which enables
CPPGC_POINTER_COMPRESSION and requires a 48 GB virtual memory
reservation (2×16 GB + 16 GB alignment).  HermitOS has no demand
paging, so this reservation cannot be satisfied — V8 either OOMs
or corrupts memory when the allocation is capped.

Disable the caged heap for Hermit so V8 uses regular allocations
for Oilpan objects instead.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: max-lt

build.rs

@@ -453,6 +453,10 @@ fn build_v8(is_asan: bool) {
     // HermitOS: enable WASM with explicit bounds checks (no signal-based trap handler)
     gn_args.push("v8_enable_trap_handler=false".to_string());
     gn_args.push("v8_enable_sandbox=false".to_string());
+    // HermitOS: disable cppgc caged heap — it requires a 48 GB virtual memory
+    // reservation for pointer compression alignment, which is impossible on a
+    // unikernel without demand paging.
+    gn_args.push("cppgc_enable_caged_heap=false".to_string());
     gn_args.push("use_sysroot=false".to_string());
     // HermitOS: use V8's bundled libc++ (statically linked into librusty_v8.a)
     // so consumers don't need to provide a separate C++ stdlib for Hermit.