feat(introspect,runtime): per-actor stack surface on ActorInfo (RFC 019 §8)

- introspect::StackInfo { reserve, guard, depth_high_water,
  parks_since_shrink, shrinks } as ActorInfo.stack; re-exported at crate
  root beside ActorInfo.
- All reads lock-free: geometry from the c6 diag slot atomics, depth =
  top - hwm (the §2 sampled high-water; doc spells out sampled-not-exact
  and that 0 means never-descheduled-at-depth), counters straight off the
  §3 atomics. Coherence for the incarnation rides read_slot's existing
  generation check, same as overruns/messages_received.
- Slot::stack_introspect(): one pub(crate) tuple accessor beside the other
  counter accessors.
- Exact RSS deliberately absent per RFC (mincore = debug tooling only,
  never a runtime path); stack_shape(pid) untouched (cold-lock exact
  variant from c2).
- tests/introspect.rs: defaults surface (64 KiB reserve / 1 MiB guard /
  sampled ~32 KiB depth / gate park counted / zero shrinks) + live shrink
  counters (spike visible pre-shrink; shrinks>=1, cooldown counter reset,
  hwm reset after crossing COOLDOWN) read mid-run -- post-join the slot
  reclaim correctly hides the incarnation, which the first draft of the
  test learned the hard way.

FLAGGED (Claude-solo calls):
- Nested StackInfo struct over five flat ActorInfo fields (grain break;
  the five fields are one concern and ActorInfo is already 12 fields).
- Field names reserve/guard/shrinks (RFC says stack_reserve/stack_guard/
  shrink count; the stack_ prefix is redundant inside StackInfo).

src/runtime.rs

@@ -662,6 +662,23 @@ impl Slot {
 
     /// Read the overrun tally (Relaxed; the snapshot reads cross-thread).
     #[inline]
+    /// RFC 019 §8 — the stack introspection tuple, all lock-free:
+    /// `(reserve, guard, top, hwm, parks_since_shrink, shrink_count)`.
+    /// Geometry from the c6 diag atomics (install-time, gen-coherent under
+    /// `read_slot`'s gen check exactly like the other counters); `hwm` is the
+    /// §2 sampled high-water (lowest saved sp). All zeros before first
+    /// install.
+    pub(crate) fn stack_introspect(&self) -> (usize, usize, usize, usize, u32, u32) {
+        (
+            self.diag_stack_reserve.load(Ordering::Relaxed),
+            self.diag_stack_guard.load(Ordering::Relaxed),
+            self.diag_stack_top.load(Ordering::Relaxed),
+            self.hwm.load(Ordering::Relaxed),
+            self.parks_since_shrink.load(Ordering::Relaxed),
+            self.shrink_count.load(Ordering::Relaxed),
+        )
+    }
+
     pub(crate) fn overruns(&self) -> u64 {
         self.overruns.load(Ordering::Relaxed)
     }