test: Deflake memory estimation test and restore its RSS regression coverage

[vdusek]

test_memory_estimation_does_not_overestimate_due_to_shared_memory fails intermittently on Linux CI (e.g. this run, ratio 0.52 vs the 0.3 tolerance; previously #1288). The sharing children only map the 100 MB segment and never touch it, so its pages are the coldest in the process tree — under memory pressure the kernel reclaims them first, and every reclaimed page silently leaves the PSS readings that the phase deltas are built from.

Two changes:

1. The sharing children now fault every page of the shared segment before the measurement. Besides removing the reclaim target, this restores the test's original purpose: untouched mappings never enter RSS either, so the test passed even with PSS unavailable — the exact overcount regression it was written to catch in fix: Use PSS instead of RSS to estimate children process memory usage on Linux #1210. With this change, a forced RSS fallback fails every round by ~11x the tolerance.
2. A distorted measurement round is retried (up to 3 rounds). Transient reclaim does not repeat identically, while a genuine overcount misses the expectation several times over in every round, so the retries cannot mask it.

Verified by reproducing CI's failure signature locally under cgroup memory pressure (systemd-run -p MemoryHigh= plus a hot-page churner): 2/10 failures before, 0/50 after, 0/40 idle runs, and the forced-RSS regression still fails deterministically.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.64%. Comparing base (142ec58) to head (f907bd0).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2202      +/-   ##
==========================================
- Coverage   93.74%   93.64%   -0.11%     
==========================================
  Files         181      181              
  Lines       12852    12852              
==========================================
- Hits        12048    12035      -13     
- Misses        804      817      +13     

Flag	Coverage Δ
unit	93.64% <ø> (-0.11%)	⬇️

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