docs(reviews/2026-07-03): fix P4 staleness — sweep status blocks + re-aim fuzz

Adversarial review finding P4: README §Status block (and the mid-file
callout) still said 'Slice 1 is the active build target' directly beneath
a slice-3 quickstart. The same §Status block is verbatim in
docs/QUICKSTART.md + docs/DEVELOPMENT.md, so fixing one without the
others would re-corrupt agent context on next read — swept all three.

fuzz/README.md planned for the SIP/SDP parsers that ADR-0007 abolished.
Re-aimed at the actual post-0007 hostile-byte surfaces: tap protocol
frames (existing, slice-2), RTP at the str0m input, Opus payload at the
decoder boundary, and the provider media-fork framing (step 5, deferred).

Refs docs/reviews/2026-07-03-adversarial-review.md P4. DCO-signed.

Signed-off-by: Aaron D. Lee <himself@adlee.work>

docs/QUICKSTART.md

@@ -2,11 +2,12 @@
 
 Get Rutster running and hear your own voice echoed back in under 5 minutes.
 
-> **Status:** Slice 1 (WebRTC media loopback) is the active build target.
-> If the workspace isn't on `main` yet, check the `slice-1-webrtc-loopback`
-> branch — that's where the implementation is landing task-by-task.
-> See [`docs/superpowers/specs/2026-06-28-slice-1-webrtc-loopback-design.md`](superpowers/specs/2026-06-28-slice-1-webrtc-loopback-design.md)
-> for the full design.
+> **Status:** Slices 1–3 are merged to `main`. Slice 4 (barge-in / VAD-driven playout
+> kill) is the active build target, in flight on the `slice-4-dev-a-reflex` +
+> `slice-4-dev-b-tap` branches. This quickstart exercises the slice-1 WebRTC media
+> loopback, which remains the simplest end-to-end demo on `main`. See
+> [`docs/superpowers/specs/2026-07-01-slice-4-barge-in-design.md`](superpowers/specs/2026-07-01-slice-4-barge-in-design.md)
+> for the active build target's design.
 
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