luagc: per-step timing, forced GC honoring, weirdperformance compat

- Per-step rdtsc timing: ZGCStats returns max_step_us and atomic_us
- AllocBench /zgcstats frame shows step/atomic timing with color coding
- /gccompare command for timing forced GC cycles
- Honor lua_setgcthreshold forced collections (collectgarbage() with low
  threshold now triggers GC instead of being silently ignored)
- Disable luaalloc in weirdperformance when luagc is active (no conflict)
- Closures go to grayagain (covers all upvalue mutation paths)

src/weirdperformance/weirdperformance.zig

@@ -326,11 +326,9 @@ pub fn installHooks() void {
     // libdeflate inflate replacement
     if (inflate_hook.install()) installed += 1;
 
-    // Lua slab allocator replacement
-    installed += luaalloc.install();
-
-    // GC phase profiler
-    installed += luagc.install();
+    // Lua slab allocator + GC: owned by luagc module.
+    // installed += luaalloc.install();
+    // installed += luagc.install();
 }
 
 pub fn lateInit() void {