fence: free and realloc stay on the fast path with the arena on - the arena's range checked inline

With the arena running every free and every realloc took the slow path (a call to check the range,
then the fence's own test). Now lp_free checks the arena's range inline and hands anything else to
libc unless the fence is tracking; lp_realloc goes slow only for a scratch request, a tracked run or
an arena block. Ready for when the final run's medians ask for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/backend/emit_fence_ir.ludic

@@ -184,10 +184,15 @@ function emit_fence_entry() -> void {
   emith("entry:\n")
   emith("  %t = load i8, ptr @lp_ft\n")
   emith("  %w = load i8, ptr @lp_want\n")
-  emith("  %a = load i8, ptr @lp_ar_on\n")
+  emith("  %x = ptrtoint ptr %o to i64\n")
+  emith("  %lo = load i64, ptr @lp_ar_lo\n")
+  emith("  %hi = load i64, ptr @lp_ar_hi\n")
+  emith("  %ge = icmp uge i64 %x, %lo\n")
+  emith("  %lt = icmp ult i64 %x, %hi\n")
+  emith("  %inar = and i1 %ge, %lt\n")
   emith("  %o1 = or i8 %t, %w\n")
-  emith("  %o2 = or i8 %o1, %a\n")
-  emith("  %on = icmp ne i8 %o2, 0\n")
+  emith("  %some = icmp ne i8 %o1, 0\n")
+  emith("  %on = or i1 %some, %inar\n")
   emith("  br i1 %on, label %slow, label %fast\n")
   emith("fast:\n")
   emith("  %p = tail call ptr @realloc(ptr %o, i64 %n)\n")
@@ -239,20 +244,17 @@ function emit_fence_entry() -> void {
   emith("}\n")
   emith("define void @lp_free(ptr %o) {\n")
   emith("entry:\n")
-  emith("  %t = load i8, ptr @lp_ft\n")
-  emith("  %a = load i8, ptr @lp_ar_on\n")
-  emith("  %o1 = or i8 %t, %a\n")
-  emith("  %on = icmp ne i8 %o1, 0\n")
-  emith("  br i1 %on, label %slow, label %fast\n")
-  emith("fast:\n")
-  emith("  tail call void @free(ptr %o)\n")
-  emith("  ret void\n")
-  emith("slow:\n")
-  emith("  %in = call i1 @lp_in_arena(ptr %o)\n")
-  emith("  br i1 %in, label %keep, label %give\n")
+  emith("  %x = ptrtoint ptr %o to i64\n")
+  emith("  %lo = load i64, ptr @lp_ar_lo\n")
+  emith("  %hi = load i64, ptr @lp_ar_hi\n")
+  emith("  %ge = icmp uge i64 %x, %lo\n")
+  emith("  %lt = icmp ult i64 %x, %hi\n")
+  emith("  %in = and i1 %ge, %lt\n")
+  emith("  br i1 %in, label %keep, label %heap\n")
   emith("keep:\n")
   emith("  ret void\n")
-  emith("give:\n")
+  emith("heap:\n")
+  emith("  %t = load i8, ptr @lp_ft\n")
   emith("  %tq = icmp ne i8 %t, 0\n")
   emith("  br i1 %tq, label %tracked, label %plain\n")
   emith("plain:\n")