test: judge page faults by whether they scale with steps, not by a flat zero

[michalharakal]

M2AcceptanceTest.m2a3_steadyStateDecodeDoesNotFaultToDisk failed on #1106's JVM leg, having passed on develop the run before. That is a flake, and it is mine — from #1042.

Why it flakes

The test asserted zero major page faults across a decode run. That is true on a quiet machine and on the reference board, but the counter it reads is the process's: on a shared CI runner, page-cache eviction under another job, or a class loaded from disk mid-run, counts against it whatever the model is doing.

What it asserts now

The property worth having is that decode does not page in per step. So the test counts faults over a short run and a long one in the same process, and requires that twelve times the steps does not mean twelve times the faults:

[m2] faults: 4 steps → 0, 48 steps → 0

Two neighbours had the same weakness and got the same treatment:

• the RSS-growth check's slack now reflects what a JIT and a GC do to a process on a loaded runner (the harness's own working set is well under a megabyte, so growth that tracks step count is still caught);
• MemoryProbeTest no longer demands that touching fresh anonymous memory produces literally zero major faults.

The strict numbers still exist

Where the environment is controlled, they are still recorded: the ARMv8.2 Cortex-A55 reference board reported 0 major faults and 0 bytes of RSS growth at both 4 and 48 steps. Those live in the memory-model documentation as measurements, which is the right place for them — a shared runner is not the instrument for that claim.

Gate

scripts/pr-gate.sh — all legs passed. The affected tests were also run three times in a row locally, green each time.

Once this lands, #1106 needs only a re-run of its JVM leg.

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