Phase 54-60: Memory-Lean mode, Balanced mode stabilization, M1 (50%) achievement

## Summary

Completed Phase 54-60 optimization work:

**Phase 54-56: Memory-Lean mode (LEAN+OFF prewarm suppression)**
- Implemented ss_mem_lean_env_box.h with ENV gates
- Balanced mode (LEAN+OFF) promoted as production default
- Result: +1.2% throughput, better stability, zero syscall overhead
- Added to bench_profile.h: MIXED_TINYV3_C7_BALANCED preset

**Phase 57: 60-min soak finalization**
- Balanced mode: 60-min soak, RSS drift 0%, CV 5.38%
- Speed-first mode: 60-min soak, RSS drift 0%, CV 1.58%
- Syscall budget: 1.25e-7/op (800× under target)
- Status: PRODUCTION-READY

**Phase 59: 50% recovery baseline rebase**
- hakmem FAST (Balanced): 59.184M ops/s, CV 1.31%
- mimalloc: 120.466M ops/s, CV 3.50%
- Ratio: 49.13% (M1 ACHIEVED within statistical noise)
- Superior stability: 2.68× better CV than mimalloc

**Phase 60: Alloc pass-down SSOT (NO-GO)**
- Implemented alloc_passdown_ssot_env_box.h
- Modified malloc_tiny_fast.h for SSOT pattern
- Result: -0.46% (NO-GO)
- Key lesson: SSOT not applicable where early-exit already optimized

## Key Metrics

- Performance: 49.13% of mimalloc (M1 effectively achieved)
- Stability: CV 1.31% (superior to mimalloc 3.50%)
- Syscall budget: 1.25e-7/op (excellent)
- RSS: 33MB stable, 0% drift over 60 minutes

## Files Added/Modified

New boxes:
- core/box/ss_mem_lean_env_box.h
- core/box/ss_release_policy_box.{h,c}
- core/box/alloc_passdown_ssot_env_box.h

Scripts:
- scripts/soak_mixed_single_process.sh
- scripts/analyze_epoch_tail_csv.py
- scripts/soak_mixed_rss.sh
- scripts/calculate_percentiles.py
- scripts/analyze_soak.py

Documentation: Phase 40-60 analysis documents

## Design Decisions

1. Profile separation (core/bench_profile.h):
   - MIXED_TINYV3_C7_SAFE: Speed-first (no LEAN)
   - MIXED_TINYV3_C7_BALANCED: Balanced mode (LEAN+OFF)

2. Box Theory compliance:
   - All ENV gates reversible (HAKMEM_SS_MEM_LEAN, HAKMEM_ALLOC_PASSDOWN_SSOT)
   - Single conversion points maintained
   - No physical deletions (compile-out only)

3. Lessons learned:
   - SSOT effective only where redundancy exists (Phase 60 showed limits)
   - Branch prediction extremely effective (~0 cycles for well-predicted branches)
   - Early-exit pattern valuable even when seemingly redundant

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scripts/soak_mixed_single_process.sh

@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Single-process soak using bench epoch mode (Phase 51).
+# - Keeps allocator state within one process.
+# - Emits per-epoch throughput + current RSS (from /proc/self/statm).
+#
+# Output CSV columns:
+#   epoch,iter,throughput_ops_s,rss_mb
+#
+# Usage:
+#   make bench_random_mixed_hakmem_minimal
+#   BENCH_BIN=./bench_random_mixed_hakmem_minimal \
+#     DURATION_SEC=300 EPOCH_SEC=5 WS=400 \
+#     ./scripts/soak_mixed_single_process.sh > soak_single.csv
+#
+# Notes:
+# - This script calibrates epoch iters from a short probe run (20M iters).
+# - For reproducibility, it forces the same "clean env" knobs as run_mixed_10_cleanenv.sh.
+
+bin=${BENCH_BIN:-./bench_random_mixed_hakmem_minimal}
+ws=${WS:-400}
+duration_sec=${DURATION_SEC:-300}
+epoch_sec=${EPOCH_SEC:-5}
+profile=${HAKMEM_PROFILE:-MIXED_TINYV3_C7_SAFE}
+
+# Clean env (same policy as scripts/run_mixed_10_cleanenv.sh).
+export HAKMEM_TINY_HEADER_WRITE_ONCE=${HAKMEM_TINY_HEADER_WRITE_ONCE:-0}
+export HAKMEM_TINY_C7_PRESERVE_HEADER=${HAKMEM_TINY_C7_PRESERVE_HEADER:-0}
+export HAKMEM_TINY_TCACHE=${HAKMEM_TINY_TCACHE:-0}
+export HAKMEM_TINY_TCACHE_CAP=${HAKMEM_TINY_TCACHE_CAP:-64}
+export HAKMEM_MALLOC_TINY_DIRECT=${HAKMEM_MALLOC_TINY_DIRECT:-0}
+export HAKMEM_FRONT_FASTLANE_ALLOC_LEGACY_DIRECT=${HAKMEM_FRONT_FASTLANE_ALLOC_LEGACY_DIRECT:-0}
+export HAKMEM_FORCE_LIBC_ALLOC=${HAKMEM_FORCE_LIBC_ALLOC:-0}
+export HAKMEM_ENV_SNAPSHOT_SHAPE=${HAKMEM_ENV_SNAPSHOT_SHAPE:-0}
+export HAKMEM_FASTLANE_DIRECT=${HAKMEM_FASTLANE_DIRECT:-1}
+export HAKMEM_FREE_TINY_FAST_MONO_DUALHOT=${HAKMEM_FREE_TINY_FAST_MONO_DUALHOT:-1}
+export HAKMEM_FREE_TINY_FAST_MONO_LEGACY_DIRECT=${HAKMEM_FREE_TINY_FAST_MONO_LEGACY_DIRECT:-1}
+
+epochs=$((duration_sec / epoch_sec))
+if (( epochs < 1 )); then
+  epochs=1
+fi
+
+probe_iters=20000000
+probe_out=$(
+  HAKMEM_PROFILE="${profile}" "${bin}" "${probe_iters}" "${ws}" 1 2>&1 \
+    | rg -o "Throughput =\\s+[0-9]+\\s+ops/s" -m 1 \
+    | rg -o "[0-9]+"
+)
+
+if [[ -z "${probe_out}" ]]; then
+  echo "failed to probe throughput" >&2
+  exit 1
+fi
+
+epoch_iters=$(( (probe_out * epoch_sec) ))
+if (( epoch_iters < 1 )); then
+  epoch_iters=1
+fi
+
+total_iters=$(( epoch_iters * epochs ))
+
+echo "epoch,iter,throughput_ops_s,rss_mb"
+
+HAKMEM_PROFILE="${profile}" \
+  HAKMEM_BENCH_EPOCH_ITERS="${epoch_iters}" \
+  "${bin}" "${total_iters}" "${ws}" 1 2>&1 \
+  | rg "^\\[EPOCH\\]" \
+  | awk '
+      {
+        # Example:
+        # [EPOCH] 0 Throughput =  12345678 ops/s [iter=500000000 ws=400] time=8.123s rss_kb=12345
+        epoch=$2;
+        thr=$5;
+        it=$7;
+        if (substr(it,1,6) == "[iter=") it=substr(it,7);
+        sub(/[^0-9].*$/, "", it);
+        rss_kb=$NF;
+        sub(/^rss_kb=/, "", rss_kb);
+        rss_mb=rss_kb/1024.0;
+        printf "%s,%s,%s,%.2f\n", epoch, it, thr, rss_mb;
+      }'