feat(hbrt): LibRegistrySnapshotAndDrain for VM pooling

Returns the lib-module + dynamic-func registry and clears it, so the same
symbol set can be installed into multiple VMs (a request pool).
RegisterLibModules drains the global registry, which would otherwise leave
only the first pooled VM with FN_HANDLE and the registered HB_FUNCs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hbrt/vm.go

@@ -60,6 +60,23 @@ func RegisterDynamicFunc(name string, fn func(*Thread)) {
 	libRegistryMu.Unlock()
 }
 
+// LibRegistrySnapshotAndDrain returns the registered lib modules and dynamic
+// functions and clears the global registry. Use to install the SAME symbol set
+// into multiple VMs (a VM pool): snapshot once, then RegisterModule /
+// RegisterSymbol into each VM. Modules hold only function pointers + metadata
+// (per-VM mutable state — statics/threads/memvars — lives on the VM), so the
+// same *Module is safe to share across VMs. Draining prevents VM.Run from
+// re-registering libModules into whichever VM happens to run first.
+func LibRegistrySnapshotAndDrain() ([]*Module, []Symbol) {
+	libRegistryMu.Lock()
+	defer libRegistryMu.Unlock()
+	mods := libModules
+	dyns := dynamicFuncs
+	libModules = nil
+	dynamicFuncs = nil
+	return mods, dyns
+}
+
 // RegisterLibModules registers any pending lib modules and dynamic functions.
 func (vm *VM) RegisterLibModules() {
 	libRegistryMu.Lock()