Add opt-in memory stability soak to the perf suite

[PeterBaker0]

Summary

Adds a long-running memory stability assessment to the performance suite, gated behind the existing opt-in PROFILE=true path.

Library / probe changes (PROFILE=true only)

• GET /_couch-auth-proxy/profile now includes process.memoryUsage() plus compact ACL cache / session resource sizes
• POST /_couch-auth-proxy/profile/gc best-effort V8 GC when the process has --expose-gc
• Profile compose overlay sets NODE_OPTIONS=--expose-gc (still opt-in via that overlay)

Perf harness

• New pnpm test:perf:memory (scripts/perf-memory.sh) runs a steady-state ACL load while sampling memory
• Trend analysis (linear fit + first/last-third median shift) budgets expected ACL-row growth and fails on unexplained heap/rss climb
• Writes test/perf/last-memory-results.json + test/perf/last-memory-report.md
• Default soak is 5 minutes (PERF_MEMORY_DURATION_SEC); normal pnpm test:perf is unchanged
• Evidence report committed at docs/memory-stability.md

Soak result (this PR)

STABLE — no evidence of a memory leak

Signal	Result
Duration	~303 s, 185k ops, 0% errors
heapUsed	16.48 → 14.93 MiB (Δ −1.55 MiB), slope ~0.45 KiB/s (r=0.094)
acl_rows	504 → 504 (plateau)
RSS	+9.6 MiB (~48 KiB/s) with flat/declining heap — native allocator retention, not a JS leak

How to run

pnpm test:perf:memory
# or longer:
PERF_MEMORY_DURATION_SEC=600 pnpm test:perf:memory