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.json

@@ -54,6 +54,16 @@
       "gc_rss_kb":     { "baseline": 13624,  "tolerance_pct": 15 },
       "bump_rss_kb":   { "baseline": 13860,  "tolerance_pct": 15 },
       "rc_rss_kb":     { "baseline": 13880,  "tolerance_pct": 15 }
+    },
+    "bench_list_sum_explicit": {
+      "gc_s":          { "baseline": 0.139,  "tolerance_pct": 10 },
+      "bump_s":        { "baseline": 0.048,  "tolerance_pct": 10 },
+      "rc_s":          { "baseline": 0.152,  "tolerance_pct": 10 },
+      "gc_over_bump":  { "baseline": 2.89,   "tolerance_pct": 8  },
+      "rc_over_bump":  { "baseline": 3.14,   "tolerance_pct": 8  },
+      "gc_rss_kb":     { "baseline": 103772, "tolerance_pct": 5  },
+      "bump_rss_kb":   { "baseline": 97636,  "tolerance_pct": 5  },
+      "rc_rss_kb":     { "baseline": 142424, "tolerance_pct": 8  }
     }
   },