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).

src/introspect.rs

@@ -91,6 +91,11 @@ pub struct ActorInfo {
     /// Chunk 2) — answers "is this actor a hotspot / draining slower than its
     /// mailbox fills." Counts received, not sent (D4). Per-incarnation (D7).
     pub messages_received: u64,
+    /// Approximate on-CPU cycles this incarnation has consumed (RFC 016 Chunk 2)
+    /// — a reductions-like work metric for relative comparison. Always 0 unless
+    /// the `budget-accounting` feature is enabled (it costs an RDTSC per resume,
+    /// D6). Per-incarnation (D7).
+    pub budget_cycles: u64,
 }
 
 /// A whole-runtime snapshot. See the module docs for the D2 tearing model.
@@ -170,6 +175,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
     let overruns = slot.overruns();
     let messages_received = slot.messages_received();
+    let budget_cycles = slot.budget_cycles();
 
     // Names + depth belong to this incarnation only if the registry mailbox's
     // pid matches the slab generation; a stale registry entry (dead prior
@@ -191,6 +197,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
         mailbox_depth,
         overruns,
         messages_received,
+        budget_cycles,
     })
 }