feat(scheduler,gen_server,gen_statem,introspect): SpawnOpts — per-actor stack shape on every spawn surface (RFC 019 §1)

SpawnOpts { stack_reserve, guard_size } with Option<usize> fields, None
resolving to the Config defaults at spawn time — a deliberate deviation
from the RFC's plain-usize struct so struct-update syntax works without
a runtime handle in scope. Threaded across the five surfaces:
spawn_with, spawn_under_with, spawn_addr_with,
GenServerBuilder::stack_opts (mirrored on NamedGenServerBuilder), and
gen_statem::spawn_with (gen_statem has no builder, so the opts ride a
_with variant — Claude-solo surface call, flagged for review). Existing
spawns forward defaults; no call-site churn.

introspect::stack_shape(pid) pulled forward (agreed) as the first slice
of the RFC 019 introspection surface, giving tests an observable.

Tests (tests/spawn_opts.rs): override/partial-override/rounding on each
surface; obligation 4 from the outside — a dead custom stack is never
handed to the next default spawn (LIFO pool would expose it), and the
reverse (default stacks ARE recycled); 8 MiB reserve behaviorally
permits ~1 MiB recursion. Also: silence unused-Result in the c1
runtime test (join now unwrapped).

src/runtime.rs

@@ -1360,7 +1360,8 @@ pub const ROOT_PID: Pid = Pid::new(u32::MAX, u32::MAX);
 // Stack acquisition / recycling — RFC 019 pool rule
 // ---------------------------------------------------------------------------
 
-/// Get a stack of the requested shape (`None` ⇒ the runtime defaults).
+/// Get a stack of the shape `opts` requests (`None` fields ⇒ the runtime
+/// defaults).
 ///
 /// Pool rule (RFC 019 §1): the pool is a uniform `Vec<Stack>` of
 /// default-shaped stacks and stays that way. Default-shaped requests try the
@@ -1369,9 +1370,10 @@ pub const ROOT_PID: Pid = Pid::new(u32::MAX, u32::MAX);
 /// lock is dropped before any mmap: no syscall ever stalls another spawner.
 pub(crate) fn acquire_stack(
     inner: &RuntimeInner,
-    shape: Option<(usize, usize)>,
+    opts: crate::scheduler::SpawnOpts,
 ) -> crate::stack::Stack {
-    let (reserve, guard) = shape.unwrap_or((inner.stack_reserve, inner.stack_guard));
+    let reserve = opts.stack_reserve.unwrap_or(inner.stack_reserve);
+    let guard = opts.guard_size.unwrap_or(inner.stack_guard);
     let default_shaped = crate::stack::round_to_pages(reserve) == inner.stack_reserve
         && crate::stack::round_to_pages(guard) == inner.stack_guard;
     if default_shaped {