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/lib.rs

@@ -83,8 +83,9 @@ pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
     block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named,
     send_after_named_wall, send_after_wall, sleep, sleep_wall,
-    spawn, spawn_addr, spawn_under, wait_readable, wait_readable_timeout, wait_writable,
-    wait_writable_timeout, yield_now, FdArm, JoinError, JoinHandle,
+    spawn, spawn_addr, spawn_addr_with, spawn_under, spawn_under_with, spawn_with,
+    wait_readable, wait_readable_timeout, wait_writable,
+    wait_writable_timeout, yield_now, FdArm, JoinError, JoinHandle, SpawnOpts,
 };
 pub use supervisor::{ChildSpec, OneForOne, Restart, Signal, Strategy};
 pub use timer::TimerId;