chore(release): v0.6.0 — RFC 019: actor stack reserve & shrink

Per-actor stack shapes on every spawn surface (SpawnOpts stack_reserve/
guard_size, Config defaults, pool rule: only default-shaped recycle);
sampled stack high-water + MADV_FREE shrink at actor-park (THRESHOLD
256 KiB, COOLDOWN 64 parks, redzone 1 page); pool-recycle MADV_DONTNEED
above the retained 64 KiB entry end; SIGSEGV overflow diagnostics
(two-tier: in-guard definitive / 1 MiB overshoot 'stepped over', prior
handler chained for foreign faults) with per-scheduler sigaltstack; and
the per-actor introspection surface (ActorInfo.stack: reserve, guard,
sampled depth, parks_since_shrink, shrinks).

Amendments ratified during implementation, for the RFC changelog:
- DEFAULT_STACK_GUARD 64 KiB -> 1 MiB, following the kernel's post-Stack-
  Clash stack_guard_gap convention; PROT_NONE width is VA-only and free.
- §7's motivating segfault was a cargo-vendored gz build, not SQLite as
  the RFC text says (cc-built C lacks -fstack-clash-protection; distro
  libraries have it — the risky class is vendored builds).
- §4 hibernate() deferred to the jar (bolt-on: force-flag on the §3
  shrink path, ~10 lines when wanted).

Gates (jobrunner box, 2026-08-08): reclaim gate PASS at c3 and again at
tip (3.0 MiB LazyFree -> kernel reclaim -> Rss to one live page ->
re-spike bit-identical, live data intact; MADV_PAGEOUT stands in for
memcg — cgroup2 is RO in the job container — driving the same reclaim
path). E1 interleaved A/B vs v0.5.0: every ka cell (the E1 subject)
within +0.3..+2.9% at tip; close-mode control cells within noise except
t8-c4 close, which is bistable (~40-44k vs ~46-49k modes for BOTH
variants, base self-disagrees by 11% across rounds); 6 rounds across two
runs are inconclusive there and a 10-round focused run is noted in the
handoff as deferred follow-up, accepted for this release.

No breaking API changes since v0.5.0: SpawnOpts fields and ActorInfo
gained members (exhaustive-construction downstream will need the new
ActorInfo.stack field; urus does not construct it).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smarm"
-version = "0.5.0"
+version = "0.6.0"
 edition = "2021"
 rust-version = "1.95"