v0.18.0

Product release for upstream Crystal ≥ 1.21 — no compiler fork.

• Finalizer fix + Linux retain=0 → fat-app RSS ~1–1.6× (Linux); Darwin acik ~90% @ ~0.63×
• Darwin Kemal tip ~84% /json @ ~1.01×
• Linux Kemal headline still v0.16 carry (~87% @ ~0.80×)
• Stack maps included but dormant (GCRY_PRECISE_STACK off)
• Opt-in GCRY_TIGHT_GROW (not default)
  Changed: Linux empty-chunk/large-cache retain default 0.
  Escape: GCRY_EMPTY_CHUNK_RETAIN / GCRY_LARGE_CACHE.
  See CHANGELOG.md.