spec(slice-4): epoch is load-bearing, not forward-compat

Propagate the 2026-07-01 two-sender revision (local VAD primary + brain
advisory secondary) into the epoch-rationale sections. §2.1, §3.2, §6.1,
§6.4 previously described barge_epoch as a forward-compat seam for a
deferred local-VAD wrapper; with LocalVadReflex in-scope this slice, the
epoch disambiguates the local-VAD primary trigger from the slower brain
advisory landing on the same event (~300 ms later).

Refs slice-4 spec §3.2, §6.1. DCO-signed.

Signed-off-by: opencode controller <controller@local>

docs/superpowers/specs/2026-07-01-slice-4-barge-in-design.md

@@ -131,12 +131,15 @@ applies the state machine and delegates to `inner.next_pcm_frame()` per the tabl
 
 The reflex owns three pieces of state:
 - `advisory_rx: mpsc::Receiver<AdvisoryEvent>` — drained sync-non-blocking via `try_recv`
-  on the 20 ms tick before delegating to `inner`. Fed by the TapEngine task over tokio mpsc.
+  on the 20 ms tick before delegating to `inner`. Fed by TWO senders on the same channel:
+  the outer `LocalVadReflex` (local VAD, the primary trigger) and the TapEngine task (the
+  brain's advisory, secondary/confirmation) — both over tokio mpsc.
 - `muted: bool` — the kill state. `next_pcm_frame` returns `None` while muted, *unless* the
   inner returns `Some` (the resume condition — the first fresh `audio_out` clears mute).
-- `barge_epoch: u64` — incremented on every `SpeechStarted`. Not strictly required for the
-  advisory-only MVP (the flush + drain makes the resume race-free), but it's the seam for a
-  future local-VAD wrapper that could race the advisory. Documented as forward-compatible.
+- `barge_epoch: u64` — incremented on every `SpeechStarted`. Load-bearing this slice: the
+  local VAD (primary) and the brain's advisory (secondary) can both fire on the same barge,
+  and the epoch distinguishes a genuine re-barge from the slower confirmation landing on the
+  same event (§3.2). The flush + drain keeps resume race-free regardless.
 
 ### 2.2 The dedicated media thread
 
@@ -241,10 +244,12 @@ un-mutes too early, brain's audio overlaps caller's next word). The resume condi
 "the brain has yielded and started a new response," which is provably signaled by the first
 `audio_out` frame after the barge — not by the caller's silence.
 
-**Why `epoch`:** not strictly needed for advisory-only (MVP), but it's the seam for the
-local-VAD backstop (deferred per §1.2). A future `LocalVadReflex` wrapper racing the
-advisory would need to disambiguate "is this barge a re-barge of the same event or a new
-one" — the epoch is the disambiguator. Forward-compatible.
+**Why `epoch`:** with two concurrent trigger sources this slice — the local VAD (primary)
+and the brain's advisory (secondary/confirmation, §6.1) — the epoch disambiguates "is this
+barge a re-barge of the same event, or a new one" when both race into the same advisory
+channel. The local VAD trips first; the brain's slower ASR-grade advisory lands ~300 ms
+later on the *same* event and must not be counted as a fresh barge. The epoch is that
+disambiguator — load-bearing now, not a forward-compat seam.
 
 ### 3.3 `AudioPipe` trait extension
 
@@ -487,7 +492,7 @@ impl<P: AudioPipe> AudioPipe for Reflex<P> {
     }
 
     fn barge_in_flush(&mut self) {
-        // Allow outer wrappers (future local-VadReflex) to barge the inner.
+        // Allow the outer `LocalVadReflex` (primary trigger) to barge the inner.
         self.inner.barge_in_flush()
     }
 }
@@ -710,15 +715,15 @@ from §6.4, which was originally specced as deferred to "when local VAD arrives.
 
 ### 6.4 Why `Reflex<P>` as a wrapper (not inline in `TapAudioPipe`)
 
-- Composition: a future `LocalVadReflex<P>` composes outside the advisory `Reflex<P>`, the
-  same way `Reflex<TapAudioPipe>` composes today. The pattern (decorator over `AudioPipe`) is
-  forward-compatible without restructuring.
+- Composition: `LocalVadReflex<P>` composes outside the advisory `Reflex<P>` (§6.1), the
+  same way `Reflex<TapAudioPipe>` composes. The pattern (decorator over `AudioPipe`) stacks
+  the two trigger sources without restructuring either.
 - The seam: `loop_driver.rs` byte-identical (still calls `pipe.next_pcm_frame()`). If the
   reflex lived inline in `TapAudioPipe`, the binary-side wiring would still change but the
   `TapAudioPipe` module itself would grow the reflex state — less isolated.
-- YAGNI caveated: the wrapper is the right abstraction for advisory-only because there's
-  exactly one reflex. When local VAD arrives, the wrapper pattern pays off; the spec does
-  not pre-empt that by collapsing the wrapper now.
+- The payoff is realized this slice, not deferred: two stacked reflexes (local-VAD primary +
+  advisory secondary) live as independent, separately-testable decorators rather than one
+  module's commingled state. Keeping them as wrappers is what makes that separation free.
 
 ### 6.5 Why `barge_in_flush` on `AudioPipe` (not just `clear_playout_ring`)