bench: 21'e — cross-language reference, AILang/C ratios

Closes the question CLAUDE.md has carried since day one ("LLVM-
linkable, performance is extremely important") with data. Hand-C
variants of the four bench fixtures, compiled with clang -O2,
each carefully matching the AILang algorithm and explicitly
documenting representation differences (cell width, leak policy)
that affect the ratio.

Three substantive findings:

1. Pure-compute parity with C: bench_compute_collatz runs at
   AILang/C = 0.99x across both allocators. AILang's IR composes
   with LLVM's optimizer at the same level a hand-C source does.
   This is the LLVM-linkable performance claim, backed by data
   for the first time. bench_compute_intsum (1.05-1.18x) confirms.

2. AILang bump beats glibc malloc 2x on linear allocation:
   bench_list_sum.bump/c = 0.50x. Bump's two-instruction inline
   fastpath outperforms glibc's free-list-managed malloc on
   no-free workloads. Quantitatively measured for the first time.

3. RC overhead vs C malloc quantified: bench_list_sum.rc/c =
   1.49x, bench_tree_walk.rc/c = 2.61x. The 8-byte refcount
   header + zero-init + libc backing add 50-160% over glibc
   malloc on these implicit-mode workloads. Explicit-mode + a
   free()-adding C variant (21'f, queued) will close the
   apples-to-apples gap on dec-cost.

CLAUDE.md updated to list bench/cross_lang.py as the third
tidy-iter gate alongside bench/check.py and bench/compile_check.py.
20 new metrics in bench/baseline_cross_lang.json with 12-15%
tolerances (cross-language ratios are inherently noisier than
within-AILang ratios — two compiler stacks contribute variance).

bench/reference/compute_collatz.c

@@ -0,0 +1,50 @@
+// Hand-C reference for bench_compute_collatz.
+//
+// Same algorithm as examples/bench_compute_collatz.ailx — for each
+// starting value in [1..N], count Collatz steps to reach 1, sum.
+// Three sizes: 10k / 100k / 500k starting values.
+//
+// Data-dependent control flow (n % 2 branch) prevents LLVM from
+// reducing this to closed form. The AILang/C wall-time ratio
+// directly reflects integer-arithmetic + branch-prediction codegen
+// quality.
+//
+// Build: clang -O2 -o compute_collatz compute_collatz.c
+// Expected stdout (one int per line):
+//   849666
+//   10753840
+//   62134795
+
+#include <stdio.h>
+
+static long collatz_steps(long n) {
+    long steps = 0;
+    while (n != 1) {
+        if ((n % 2) == 0) {
+            n = n / 2;
+        } else {
+            n = n * 3 + 1;
+        }
+        steps += 1;
+    }
+    return steps;
+}
+
+static long sum_steps(long n) {
+    long total = 0;
+    for (long i = n; i > 0; i--) {
+        total += collatz_steps(i);
+    }
+    return total;
+}
+
+static void run_one(long n) {
+    printf("%ld\n", sum_steps(n));
+}
+
+int main(void) {
+    run_one(10000);
+    run_one(100000);
+    run_one(500000);
+    return 0;
+}