fix(sweep): default env pass to process-per-sample (max-tasks-per-child=1) to end EGL contact-arena overflow

[KE7]

Problem

The validation sweep's ENV pass (G5/G6 create a LIBERO env + EGL render context + step MuJoCo) recurrently hit the mjWARN_CONTACTFULL / ncon=5000 contact-buffer overflow. Root cause (validation_run2/rca + PR #18 d.4): EGL render-context state and the MuJoCo contact arena (ncon) ACCUMULATE across simulation samples within a single long-lived worker process and eventually overflow.

Fix (scoped, minimal — only the DEFAULT changes)

Make the effective default of --max-tasks-per-child conditional on pass type:

• ENV pass (scenic_only=False) → default 1 = process-per-sample. With chunk_size = workers * 1, every worker is torn down + respawned after exactly ONE condition, so each simulation sample gets a fresh EGL/MuJoCo process and state cannot accumulate.
• --scenic-only pass (G0–G3, pure-Python, no env/EGL) → keeps the higher DEFAULT_MAX_TASKS_PER_CHILD = 64 for speed (no GL/sim state to accumulate).

Detection of omitted-vs-explicit: argparse default is now None; resolve_max_tasks_per_child(explicit, scenic_only=...) resolves it. An explicit --max-tasks-per-child always overrides both passes.

The batch-of-pools chunking/teardown logic is unchanged — only the default N differs. No behavior change beyond the default; no masking.

Files

• src/libero_infinity/validation/sweep.py: new ENV_MAX_TASKS_PER_CHILD=1 + resolve_max_tasks_per_child(); argparse default=None + updated help text citing the root cause; main() resolves before run_sweep.
• tests/test_sweep_pool_recycle.py: Tier-1 (no-sim) tests — env default=1, scenic-only default=64, explicit override on both passes, and end-to-end argparse omitted-vs-explicit wiring.

CI safety

The Validation Smoke (G0–G3) job runs --scenic-only, so it resolves to 64 (unchanged) — process-per-sample does NOT affect the CI time budget. Verified locally: smoke summary reports max_tasks_per_child: 64, 6/6 rows G0–G3 pass.

Local CI (all green)

• ruff check + ruff format --check + black --check on src/ tests/ ✅
• Tier-1 6-file pytest + sweep tests: 189 passed ✅
• Validation smoke G0–G3: 6/6 pass ✅

🤖 Generated with Claude Code