bench: 21'b — closure-pair + HOF/poly fixtures, 47-metric baseline

Two new throughput fixtures targeting blind spots in the 21'a
corpus:

- bench_closure_chain exercises the build_pair_drop_fn codegen
  path (the 18c.4 doubled-braces trigger). Each iteration of
  run_loop allocates a {thunk, env} closure pair via the
  let-rec-name-as-value escape route. Sizes 10k / 100k / 500k.
  rc/bump = 4.14x — materially worse than the 2.91x / 2.59x of
  the linear/tree fixtures, exposing that closure work pays the
  RC alloc tax twice (pair + env-struct).

- bench_hof_pipeline exercises poly-ADT instantiation and
  indirect dispatch via fold_with_fn over List<a>. Sizes 100k /
  1M / 3M elements. Ratios essentially match bench_list_sum,
  confirming the 13b static-template-plus-ctor-inline design
  has zero measurable overhead at this scale.

Baseline file extends from 31 to 47 metrics. The two new fixtures
build clean under all three allocators; the rc-arm build exercises
the per-type drop fn for the closure-pair, providing a tripwire
for any future 18c.4-class IR malformedness.

JOURNAL records both surprises (4.14x closure tax, ~zero HOF/poly
overhead) and explicitly notes the dispersion observation on
explicit_at_rc.p99 — three captures today (357.5 / 294.6 / 251.5)
confirm wide run-to-run variance on that fixture. Methodology
upgrade (n>=10 captures or tighter fixture) deferred to 21'c.

bench/run.sh fixtures array updated. bench/check.py needed no
changes — its parser handles the wider table by metric name.

bench/baseline.json

@@ -24,6 +24,26 @@
       "gc_rss_kb":     { "baseline": 73260,  "tolerance_pct": 5  },
       "bump_rss_kb":   { "baseline": 55208,  "tolerance_pct": 5  },
       "rc_rss_kb":     { "baseline": 108956, "tolerance_pct": 5  }
+    },
+    "bench_closure_chain": {
+      "gc_s":          { "baseline": 0.013,  "tolerance_pct": 25 },
+      "bump_s":        { "baseline": 0.007,  "tolerance_pct": 25 },
+      "rc_s":          { "baseline": 0.029,  "tolerance_pct": 20 },
+      "gc_over_bump":  { "baseline": 1.86,   "tolerance_pct": 15 },
+      "rc_over_bump":  { "baseline": 4.14,   "tolerance_pct": 15 },
+      "gc_rss_kb":     { "baseline": 13688,  "tolerance_pct": 15 },
+      "bump_rss_kb":   { "baseline": 15836,  "tolerance_pct": 15 },
+      "rc_rss_kb":     { "baseline": 39644,  "tolerance_pct": 10 }
+    },
+    "bench_hof_pipeline": {
+      "gc_s":          { "baseline": 0.134,  "tolerance_pct": 10 },
+      "bump_s":        { "baseline": 0.048,  "tolerance_pct": 10 },
+      "rc_s":          { "baseline": 0.136,  "tolerance_pct": 10 },
+      "gc_over_bump":  { "baseline": 2.79,   "tolerance_pct": 8  },
+      "rc_over_bump":  { "baseline": 2.83,   "tolerance_pct": 8  },
+      "gc_rss_kb":     { "baseline": 103788, "tolerance_pct": 5  },
+      "bump_rss_kb":   { "baseline": 97448,  "tolerance_pct": 5  },
+      "rc_rss_kb":     { "baseline": 193640, "tolerance_pct": 5  }
     }
   },