bench: 21'f — explicit-mode pair, full alloc+dec vs malloc+free

Closes the apples-to-apples gap from 21'e. Adds:
- examples/bench_list_sum_explicit.ailx — same algorithm and sizes
  as bench_list_sum, fully (borrow)/(own)/(drop-iterative)
  annotated so codegen emits proper inc/dec instrumentation.
- bench/reference/list_sum_explicit_free.c — same algorithm
  with explicit free() walking the chain after sum.

The full alloc+dec vs malloc+free comparison reveals two non-
trivial conclusions:

1. AILang's full RC pipeline is only 26% slower than glibc
   malloc+free on this workload (rc/c = 1.26x). The implicit-
   mode comparison's 1.42x was misleading — it counted neither
   pipeline's free path. The fair ratio is 1.26x, materially
   better than the previous read.

2. RC's dec is cheaper per cell than glibc free(). AILang
   dec-tax: ~3 ns/cell. C free-tax: ~5.5 ns/cell. Plausible
   cause: ailang_rc_dec operates on a known-shape cell with a
   fixed-offset refcount and a static per-type drop fn — no
   free-list bucketing, no header introspection, no global lock.

bump's advantage expresses fully: bench_list_sum_explicit.bump/c
= 0.42x means AILang at bump is 2.4x faster than C malloc+free.
Sets a useful upper bound on a slab/pool RC allocator's potential.

The 21'-family arc — bench-regression infrastructure — is now
substantively complete: 21'a (bench/check.py), 21'b (corpus
widening), 21'c (compile_check.py), 21'd (pure-compute fixtures
+ harness hardening), 21'e (cross-language hand-C), 21'f (explicit
apples-to-apples). 63 runtime metrics + 18 compile metrics + 25
cross-lang metrics under regression coverage. Any future iter
that regresses any axis beyond tolerance gets caught at the next
family close.

Remaining queue is back to substantive language work — Family 21
(typeclasses / polymorphic ADTs at runtime / pattern-binding
generalisation) is now an orchestrator-level fork that needs
direct user input.

bench/baseline_cross_lang.json

@@ -6,89 +6,111 @@
   "fixtures": {
     "bench_list_sum": {
       "ail_rc_s": {
-        "baseline": 0.141597,
+        "baseline": 0.138906,
         "tolerance_pct": 15
       },
       "ail_bump_s": {
-        "baseline": 0.048038,
+        "baseline": 0.050245,
         "tolerance_pct": 15
       },
       "c_s": {
-        "baseline": 0.095312,
+        "baseline": 0.09749,
         "tolerance_pct": 15
       },
       "rc_over_c": {
-        "baseline": 1.485614,
+        "baseline": 1.42483,
         "tolerance_pct": 12
       },
       "bump_over_c": {
-        "baseline": 0.504003,
+        "baseline": 0.515393,
         "tolerance_pct": 12
       }
     },
     "bench_tree_walk": {
       "ail_rc_s": {
-        "baseline": 0.097028,
+        "baseline": 0.100173,
         "tolerance_pct": 15
       },
       "ail_bump_s": {
-        "baseline": 0.038905,
+        "baseline": 0.038673,
         "tolerance_pct": 15
       },
       "c_s": {
-        "baseline": 0.037159,
+        "baseline": 0.036567,
         "tolerance_pct": 15
       },
       "rc_over_c": {
-        "baseline": 2.611185,
+        "baseline": 2.739449,
         "tolerance_pct": 12
       },
       "bump_over_c": {
-        "baseline": 1.046998,
+        "baseline": 1.057606,
         "tolerance_pct": 12
       }
     },
     "bench_compute_intsum": {
       "ail_rc_s": {
-        "baseline": 0.00044,
+        "baseline": 0.000362,
         "tolerance_pct": 15
       },
       "ail_bump_s": {
-        "baseline": 0.000393,
+        "baseline": 0.000371,
         "tolerance_pct": 15
       },
       "c_s": {
-        "baseline": 0.000374,
+        "baseline": 0.000361,
         "tolerance_pct": 15
       },
       "rc_over_c": {
-        "baseline": 1.177715,
+        "baseline": 1.002676,
         "tolerance_pct": 12
       },
       "bump_over_c": {
-        "baseline": 1.050856,
+        "baseline": 1.028255,
         "tolerance_pct": 12
       }
     },
     "bench_compute_collatz": {
       "ail_rc_s": {
-        "baseline": 0.056878,
+        "baseline": 0.057994,
         "tolerance_pct": 15
       },
       "ail_bump_s": {
-        "baseline": 0.056547,
+        "baseline": 0.057159,
         "tolerance_pct": 15
       },
       "c_s": {
-        "baseline": 0.05748,
+        "baseline": 0.05776,
         "tolerance_pct": 15
       },
       "rc_over_c": {
-        "baseline": 0.989523,
+        "baseline": 1.00405,
         "tolerance_pct": 12
       },
       "bump_over_c": {
-        "baseline": 0.983756,
+        "baseline": 0.989593,
+        "tolerance_pct": 12
+      }
+    },
+    "bench_list_sum_explicit": {
+      "ail_rc_s": {
+        "baseline": 0.150754,
+        "tolerance_pct": 15
+      },
+      "ail_bump_s": {
+        "baseline": 0.0495,
+        "tolerance_pct": 15
+      },
+      "c_s": {
+        "baseline": 0.119204,
+        "tolerance_pct": 15
+      },
+      "rc_over_c": {
+        "baseline": 1.264667,
+        "tolerance_pct": 12
+      },
+      "bump_over_c": {
+        "baseline": 0.415253,
         "tolerance_pct": 12
       }
     }