luagc: working incremental sweep via list truncation (5s -> 120ms worst)

Rootgc sweep chunked by temporarily NULLing a next pointer and calling
the original lua_gc_remove_objects on the truncated sublist. Swept
survivors are detached to a separate list to prevent re-sweeping.
Lists reconnected after final chunk.

Birth-mark via binary patch of luaC_link immediate byte (0x6F7B37)
ensures new objects born during sweep survive.

Profiling shows sweep chunks at 0-9ms each (was 225ms avg, 5s worst).
Remaining bottleneck: atomic mark (80ms) + udata/string sweep (44-92ms).

src/weirdperformance/weirdperformance.zig

@@ -329,8 +329,8 @@ pub fn installHooks() void {
     // Lua slab allocator replacement
     // installed += luaalloc.install(); // disabled for testing
 
-    // Incremental GC -- disabled, needs more research
-    // installed += luagc.install();
+    // GC phase profiler
+    installed += luagc.install();
 }
 
 pub fn lateInit() void {
@@ -340,7 +340,8 @@ pub fn lateInit() void {
 
 pub fn removeHooks() void {
     if (g_is_hook_owner) {
-        luaalloc.dumpStats(); // dump size histogram before unhooking
+        luaalloc.dumpStats();
+        luagc.dumpStats();
         filecache.remove();
         inflate_hook.remove();
         transform_hook.detach();