v0.9.0

Added

• Process-GC parallel mark (STW-exempt): with GCRY_PARALLEL_MARK=N / parallel_mark_workers > 1, helpers are raw LibC.pthread_create threads (not Crystal::Thread), so stop_world does not suspend them. They steal grey objects under @mark_lock (parallel_mark_stolen). Fork child abandons the pool via reset_mark_workers_after_fork.
• Library-heap parallel mark: with parallel_mark_workers > 1 and stop_the_world == false, helper Threads steal grey objects (parallel_mark_stolen).
• Stack scrubbing (no Crystal patch): GCRY_CLEAR_STACK=1 zeros a window below SP (skips x86_64 red zone; default every 16 allocs) without calling Fiber/Thread APIs; GCRY_SCRUB_FIBERS=1 zeros a capped window below each parked fiber's saved SP before mark (not the full unused stack — that faults pages in and blows RSS). Metrics: clear_stack_* / fiber_scrub_* (json_stats + Prometheus). Not stack maps; measure before enabling as default.
• Richer Gcry::Observability.json_stats (phase timers, mapped/live bytes, TLAB, parallel-mark, barrier) — Kemal /gc-stats uses it.
• Prometheus: TLAB, parallel-mark, phase, layout, SP clamp, barrier, size-class live / released chunk gauges; gcry_clear_stack_* / gcry_fiber_scrub_*.
• Median-of-3 helpers: bench/median_kemal_boehm.sh, bench/median_acikturkiye_boehm.sh.

Changed

• README / HARDENING / POLICY: GCRY_PARALLEL_MARK is real for process GC (pthread steals), not counter-only — and labeled experimental / measure first (Kemal /json + acikturkiye /api/v1/ thr regressed vs N=1 in same-host wrk).
• README / HARDENING: document GCRY_DISABLE_* escapes, GCRY_TLAB, stack-scrub knobs.
• Dogfood docs: docs/ACIKTURKIYE.md + docs/API.md point at Observability routes; acikturkiye make run-demo-gcry / README GC section.
• Same-host Kemal (0.9.0 cut, median of 3, scrub off): / ~89% of Boehm; /json ~92%; post-GC RSS ~0.97× — see docs/PERF.md.
• Same-host acikturkiye /api/v1/ (median of 3, scrub off): thr trial-median ~93%; post-GC RSS ~2.84× (was ~3.20× at 0.8.0) — see docs/ACIKTURKIYE.md.

Fixed

• clear_stack aarch64 SEGV: wipe used approximate pointerof(local) as SP (mid-frame). With no x86_64 red zone that zeroed the leaf frame (Invalid memory access @ 0x0 on CI test (aarch64 native)). Now reads hardware SP (Roots.hardware_stack_pointer) plus a leaf margin.