RFC 016 Chunk 2c: approximate per-actor time-budget (reductions-like)

Accumulate on-CPU cycles per actor as ActorInfo.budget_cycles, behind
the off-by-default budget-accounting feature (D6) — a reductions-style
work metric for relative comparison across runs.

- Approximate by design (per Mark): charge now - slice-start once at the
  yield point, reusing the timestamp reset_timeslice already sets, so
  one RDTSC per resume not two. Wake-slot resumes inherit the slice and
  so slightly over-attribute the chain's time to the woken actor — noise
  that averages out; we trade exactness for half the hot-path cost.
- Field/ActorInfo member are unconditional (keeps the snapshot shape
  stable across the feature flag, D1); only the accumulation is gated,
  so default builds are byte-identical and pay nothing. Reads return 0
  when off. Single-writer Relaxed like the other counters; reset in
  reset_counters (D7).

Matrix: default + feature-on + rq-mpmc + rq-striped + release + trace all
green; loom unaffected (feature off under --cfg loom).

Cargo.toml

@@ -10,6 +10,11 @@ unexpected_cfgs = { level = "warn", check-cfg = ["cfg(loom)"] }
 [features]
 default = ["rq-mutex"]
 smarm-trace = []
+# 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
+# unless this is enabled.
+budget-accounting = []
 # Run-queue selection: exactly one, compile-time (see src/run_queue.rs).
 # Non-default variants need --no-default-features (features are additive).
 rq-mutex = []