core: add a state dump to tools/bench load on task timeout

Per-task engine state, downloaded/effective_segments, every segment's own
state, and the engine's SegmentBudget snapshot -- printed once, when a task
times out, instead of needing to re-run the bench under a debugger or add
throwaway instrumentation to find out why. This is what actually diagnosed
the SegmentBudget over-admission bug fixed in the previous commit: the dump
showed budget.active pinned at max_active_segments while multiple tasks sat
starved, which is what pointed straight at confirm_slot()'s missing
engine-wide check rather than a per-task target bug.

Also updates the vdm_bench_load20 ctest registration's comment: the
straggler under --preset tsan that motivated running it at reduced
concurrency (docs/adr/0016's postscript) is now suspected to have been the
same SegmentBudget bug (docs/adr/0017), not the TSan-timing artifact first
guessed -- not reverified at the DoD's full shape under TSan in this change,
so the reduced-concurrency registration stays for now.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Q3QrF7rCt21bkAjt9BCDFQ

tools/bench/CMakeLists.txt

@@ -37,17 +37,18 @@ if(VELOX_BUILD_TESTS)
     set_tests_properties(vdm_bench_throughput_smoke PROPERTIES LABELS "bench" TIMEOUT 120)
 
     # --tasks 8 --segments 2 (not the DoD's 20 tasks * default_segments=8 = 160 concurrent
-    # segments): at the full shape, TSan's per-access instrumentation overhead was observed
-    # to leave a straggler task not just slow but still incomplete past a 300s-per-task
-    # budget -- reproduced at tasks=20/segments=8 (2 stragglers) and, smaller but still
-    # present, at tasks=20/segments=2 (1 straggler); tasks=8/segments=2 (16 concurrent
-    # connections) was reliable across repeated runs. No TSan report ever accompanied a
-    # straggler (this isn't a race -- see docs/adr/0016's postscript), so it reads as some
-    # combination of TSan's overhead and this environment's scheduling, not an engine bug;
-    # still, "every task completes" is exactly what this smoke test is supposed to check
-    # (see the split above), so the bar it runs at has to be one that actually holds. The
-    # DoD's real 20-task/default-segments/60MB-RSS shape is exercised by the manual/CI M7
-    # sign-off run in this file's header comment, at --preset release, where it passes.
+    # segments): at the full shape under --preset tsan, a straggler task was observed not
+    # just slow but still incomplete past a 300s-per-task budget (docs/adr/0016's
+    # postscript, written before docs/adr/0017's SegmentBudget fix landed) -- now suspected
+    # to have been that same over-admission bug (a segment denied a slot with nothing to
+    # wake it), not a TSan-timing artifact as first guessed, since the symptom -- a task
+    # that simply never resumes -- matches exactly. Left at this reduced concurrency rather
+    # than reverting: re-verifying the full shape is clean under TSan wasn't done as part
+    # of that fix (see docs/adr/0017's "Consequences"), so this is still the bar that's
+    # actually known to hold. "Every task completes" is what this smoke test exists to
+    # check (see the split above); the DoD's real 20-task/default-segments/60MB-RSS shape
+    # is exercised by the manual/CI M7 sign-off run in this file's header comment, at
+    # --preset release, where it passes (core/docs/m7-baseline.md).
     add_test(NAME vdm_bench_load20 COMMAND vdm_bench load --tasks 8 --task-size 2M --segments 2)
     set_tests_properties(vdm_bench_load20 PROPERTIES LABELS "bench" TIMEOUT 300)