feat(causal): native causal profiling behind smarm-causal (RFC 007 v1)

causal_site! scoped site guards per actor slot, progress! throughput
points, and a Coz-style virtual-speedup engine hooked into
maybe_preempt's amortized cold block: target-site samples grow a global
delay ledger; bystanders spin-absorb their debt at the next causal
check, with timeslice extension so injected delay is not charged
against the slice.

Resume credit (Coz's blocked-thread rule) is gated on a causal_parked
slot bit set only by a real park: crediting on every resume made any
yield-cadence actor delay-immune and every experiment inert (found
live on a 24-core run — dead-flat deltas across all sites).

Report normalization uses a measured TSC frequency (~50ms calibration
on first use) instead of the crate-wide 3 GHz assumption, which
uniformly inflated impact numbers on a 3.7 GHz box. impact_pct() is
the machine-readable form of the summary for programmatic checks.

examples/causal_pipeline.rs burns fixed *work* (calibrated LCG loop),
not fixed wall time — a timed busy-wait absorbs injected delay into
its own budget and reads as a no-op. Self-checking: exits nonzero if
causal separation fails; skips the verdict below 4 cores. Validated
on a 24-core box: reserve (true bottleneck) +29.3%@25/+83.5%@50;
serialize and background-compaction ~0%.

Known v1 gaps (jar): timer-heap deadlines unshifted, no-check!/no-alloc
actors undelayable, multi-scheduler coherence best-effort Relaxed,
off-CPU blame punted, Instant::now() uncorrected.

Zero-cost with the feature off; clippy -D warnings clean both ways;
full suite green with and without smarm-causal.

Cargo.toml

@@ -19,6 +19,10 @@ expect_used = "deny"
 [features]
 default = ["rq-mutex"]
 smarm-trace = []
+# RFC 007: native causal profiling. Zero cost when off (cf. smarm-trace): the
+# hook in `maybe_preempt` and the resume-path fast-forward compile away; the
+# two Slot ledger fields exist regardless and stay 0 (budget_cycles precedent).
+smarm-causal = []
 # RFC 016 Chunk 2: cycle-accurate per-actor time-budget accounting. Off by
 # default — it costs two extra RDTSC reads per actor resume on the hot path
 # (D6). The `ActorInfo.budget_cycles` field exists regardless; it just stays 0
@@ -89,3 +93,7 @@ harness = false
 [[example]]
 name = "observer"
 required-features = ["observer"]
+
+[[example]]
+name = "causal_pipeline"
+required-features = ["smarm-causal"]