feat(causal): timer-heap virtual time — deadlines chase injected delay (RFC 007)

Injected delays dilate virtual time for the workload, but timer deadlines
stayed wall-anchored: a sleep or receive-timeout fired early in virtual
terms, so timeout/retry behaviour sped up relative to the dilated world
(v1 known gap #1).

Every heap Entry now carries delay_stamp — the global delay ledger at
(re-)queue time, cfg-gated on smarm-causal. pop_due converts any debt
accrued since the stamp to wall time (tsc_hz) and shifts the effective
deadline; a not-yet-due entry is re-queued at the shifted deadline with a
fresh stamp, so it keeps chasing delay injected while it waits. seq is
preserved across re-queues, keeping send_after cancellation identity
intact (cancelled entries are discarded before any shift). Zero debt is
byte-identical to the old path; peek_deadline may under-report, costing
one spurious scheduler wake per injected chunk (documented).

This also makes the park-gated resume credit *correct* rather than
forgiving for sleepers: a sleeping actor now physically pays its debt by
sleeping longer, so the on_resume fast-forward reflects real payment
(sleeping_actor_pays_injected_delay pins this end-to-end through the
runtime).

New test hooks: inject_delay_cycles_for_test (deterministic ledger
driver, eagerly TSC-calibrating so conversion never stalls a scheduler
loop) and cycles_to_duration. The ledger is process-global, so the
delta-sensitive causal tests now serialize on a shared test mutex — they
were racy under the parallel test harness before this, in principle.

src/causal.rs

@@ -397,6 +397,20 @@ mod inner {
         end();
     }
 
+    /// Test support: grow the global delay ledger directly, as if target-site
+    /// samples had attributed `cycles` — deterministic driver for the timer
+    /// virtual-time tests. Calibrates the TSC eagerly so conversion later
+    /// never stalls a scheduler loop.
+    pub fn inject_delay_cycles_for_test(cycles: u64) {
+        let _ = tsc_hz();
+        GLOBAL_DELAY.fetch_add(cycles, Ordering::Relaxed);
+    }
+
+    /// Convert ledger cycles to wall time at the measured TSC rate.
+    pub fn cycles_to_duration(cycles: u64) -> Duration {
+        Duration::from_secs_f64(cycles as f64 / tsc_hz())
+    }
+
     /// Controller parameters: which speedups to try per site, and the
     /// experiment/cooldown windows.
     pub struct ExperimentPlan {