feat(causal): wall-anchored send_after — user-facing timer opt-out (RFC 007)

send_after_wall / send_after_named_wall (+ Timers::insert_send_wall) arm a
message-delivery timer that opts out of the RFC 007 virtual-time shift and
fires at its raw deadline regardless of injected delay — the Send-reason
sibling of sleep_wall, closing the jar item whose substrate efbc254 landed.
For deadlines that reflect the outside world (protocol timeouts, wall-clock
schedules) rather than workload pacing. cancel_timer is anchor-agnostic and
unchanged; without the feature the API exists and is identical to send_after.

The gen_server timer layer (send_after_to, RFC 015 §5) deliberately stays
virtual-only — an opt-out there means new options on the gen_server/statem
timeout API, out of scope for now.

Tests: wall send fires at raw deadline while a virtual sibling shifts;
cancel on a wall send with debt outstanding; featureless delivery/cancel
smokes through the public named API. 15/15 causal, 34/34 binaries both
feature configs, lib clippy clean both.

src/lib.rs

@@ -80,7 +80,8 @@ pub use registry::{
 };
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
-    block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named, sleep, sleep_wall,
+    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,
 };