fix(bench): gate oversubscribed C>=4 unbounded shapes behind a define

bench_unbounded was hanging in CI on the unbounded_sipmuc 1p4c shape:
4 consumers compete in a strict round-robin slot order (each consumer
must take its turn before another can pop), and on a 4-vCPU runner
one descheduled consumer blocks every other. The same starvation
pattern hits every unbounded mupmuc shape with C>=4.

Add `-d:BenchSkipOversubscribed` (set in PR CI bench.yml) that drops:

  - unbounded_sipmuc/mpmc_unbounded/1p4c
  - unbounded_mupmuc/mpmc_unbounded/{1p4c, 2p4c, 4p1c, 4p2c, 4p4c}

mupsic shapes (multi-producer, single consumer) are kept regardless:
the round-robin ordering exists on consumers only, so a single consumer
cannot starve. mupsic 4p1c is also in tests/fixtures/pre-split-slugs.json
and the deletion-safety guard would fail without it.

The full grid still runs in `bench-comparison.yml` nightly cron / on
workflow_dispatch where a beefier runner is available. PR CI now gets
the meaningful subset (4 mupmuc shapes covering each P,C in {1,2})
that fits a 4-vCPU runner without hanging.

Author: elijahr

.github/workflows/bench.yml

@@ -137,6 +137,7 @@ jobs:
                 -d:UnboundedMupmucMessageCount=50000 \
                 -d:UnboundedMupmucRuns=2 \
                 -d:BenchUnboundedWarmup=1 \
+                -d:BenchSkipOversubscribed \
                 benchmarks/nim/bench_unbounded.nim
               ;;
             bench_latency)

benchmarks/nim/bench_unbounded.nim

@@ -441,35 +441,48 @@ proc runVariant(variant: string, em: var BMFEmitter) =
       UnboundedSipmucMessageCount)
     runUSipmucShape[2](em, UnboundedSipmucRuns, BenchUnboundedWarmup,
       UnboundedSipmucMessageCount)
-    runUSipmucShape[4](em, UnboundedSipmucRuns, BenchUnboundedWarmup,
-      UnboundedSipmucMessageCount)
+    when not defined(BenchSkipOversubscribed):
+      runUSipmucShape[4](em, UnboundedSipmucRuns, BenchUnboundedWarmup,
+        UnboundedSipmucMessageCount)
   of "unbounded_mupsic":
+    # mupsic shapes are kept under BenchSkipOversubscribed: a single
+    # consumer cannot trigger the round-robin starvation that hangs
+    # sipmuc/mupmuc with C >= 4, and `mpsc_unbounded/4p1c` is in the
+    # pre-split deletion-safety fixture.
     runUMupsicShape[1](em, UnboundedMupsicRuns, BenchUnboundedWarmup,
       UnboundedMupsicMessageCount)
     runUMupsicShape[2](em, UnboundedMupsicRuns, BenchUnboundedWarmup,
       UnboundedMupsicMessageCount)
     runUMupsicShape[4](em, UnboundedMupsicRuns, BenchUnboundedWarmup,
       UnboundedMupsicMessageCount)
   of "unbounded_mupmuc":
-    # {1,2,4} P x {1,2,4} C grid (9 shapes).
+    # {1,2,4} P x {1,2,4} C grid (9 shapes). Shapes with P + C > 4
+    # hang under 4-vCPU oversubscription: the round-robin consumer
+    # ordering in unbounded_sipmuc/mupmuc forces strict per-shape
+    # turn-taking, so a single descheduled consumer blocks every
+    # other consumer. `-d:BenchSkipOversubscribed` (set in PR CI)
+    # drops the offending shapes; the full grid still runs in
+    # `bench-comparison.yml` nightly cron / on workflow_dispatch
+    # where a beefier runner is available.
     runUMupmucShape[1, 1](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
       UnboundedMupmucMessageCount)
     runUMupmucShape[1, 2](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
       UnboundedMupmucMessageCount)
-    runUMupmucShape[1, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
-      UnboundedMupmucMessageCount)
     runUMupmucShape[2, 1](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
       UnboundedMupmucMessageCount)
     runUMupmucShape[2, 2](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
       UnboundedMupmucMessageCount)
-    runUMupmucShape[2, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
-      UnboundedMupmucMessageCount)
-    runUMupmucShape[4, 1](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
-      UnboundedMupmucMessageCount)
-    runUMupmucShape[4, 2](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
-      UnboundedMupmucMessageCount)
-    runUMupmucShape[4, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
-      UnboundedMupmucMessageCount)
+    when not defined(BenchSkipOversubscribed):
+      runUMupmucShape[1, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
+        UnboundedMupmucMessageCount)
+      runUMupmucShape[2, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
+        UnboundedMupmucMessageCount)
+      runUMupmucShape[4, 1](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
+        UnboundedMupmucMessageCount)
+      runUMupmucShape[4, 2](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
+        UnboundedMupmucMessageCount)
+      runUMupmucShape[4, 4](em, UnboundedMupmucRuns, BenchUnboundedWarmup,
+        UnboundedMupmucMessageCount)
   else:
     raise newException(ValueError, "unknown variant: " & variant)