test(engine): drop non-portable 5k EPS floor in DurationBound (IB-21)

[miladaslaner]

What & why (IB-21, Bug — release-blocker)

TestRun_DurationBound/5k_eps blocked the v0.4.1 release: its sustained-EPS lower floor (generated ≥ 0.85× eps·after) is not portable to a 2-core GitHub Actions runner, which cannot physically push 5k EPS through generate→pipeline→send (CI ratio ~0.34 → did not sustain EPS), yet passes on a fast dev box (~1.04). This is a test-portability defect — the IB-19 engine fix is correct and unchanged.

Change (founder-confirmed Option (c) — test-only)

• Removed the 5k subtest's sustained-EPS lower-floor assertion (old engine_test.go:528-531) and its now-dead minGenRatio const. No capability-probe / measured-send-rate / skip-with-log machinery.
• Retained the sustained-EPS floor only at the 1k subtest (10 req/s — trivially sustainable on 2 cores).
• Preserved the IB-19 stop.after overrun guard (wall ≤ after+1.0s AND generated ≤ 1.15× eps·after) universally at BOTH 1k and 5k, tolerances unchanged.
• Added an AC4 governance comment at both overrun blocks: these are the IB-19 regression guard and must not be hidden behind testing.Short() (both ci.yml:58 and release.yml:35 run this test under -race -short).

Out of scope (unchanged): internal/engine product code, IB-19, overrun tolerances, ci.yml/release.yml flags.

Evidence

• Fail-before (overrun guard, be2d195 pre-IB-19 engine, current test overlaid): 1k wall=9.248s / ratio 3.63×, 5k wall=6.021s / ratio 1.32× → overrun assertions FAIL (5k fails via the upper bound even with the floor removed, proving the guard is preserved).
• Pass-after (branch): go test -race ./internal/engine -run TestRun_DurationBound -v → 1k ratio 1.10, 5k ratio 1.03, both PASS.
• Authoritative gate go test -race -short ./... (== ci.yml/release.yml): 28 packages ok, 0 FAIL, exit 0.
• go build ./... ✓ · go vet ./... ✓ · gofmt -l . clean · golangci-lint run ./internal/engine/... → 0 issues.

(The non--short A1a/A1b throughput acceptance benchmarks in bench_a1_test.go fail on constrained/loaded hardware by design — they need ~100k EPS on a fast dedicated box and are -short-skipped in every CI/release gate. That file is untouched here.)

Diff: internal/engine/engine_test.go only (test-only).

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