docs: phase-0 — UDP phase model vs alktty-style demux; fork-vs-reimplement collapses onto OQ-SK-04

- OQ-SK-03: RFC 1928 control/data never interleave (fast-socks5
  wait_on_tcp treats post-reply control bytes as garbage), so the
  alktty 5-stream demux solves a problem SOCKS5 doesn't have; phase
  model (pass-through CONNECT, len-prefixed datagrams post-reply)
  subsumes it; the demux's real advantage is an additive in-band
  stream-type vocabulary — noted as the honest one-way-door fork
- OQ-SK-04: no structural fork needed for the channels surface
  (router.rs interception shape covers CONNECT; run_udp_proxy_custom
  + reply_success + the pure UDP codec cover ASSOCIATE); the
  fork/wrap/reimplement decision is driven entirely by the wasm
  feature-gating outcome; full fork documented as least attractive

docs/research/phase-0.md

@@ -361,11 +361,43 @@ the datagram stage must ride a channel. Sub-questions:
   sentinel-reply shape is fine and no virtualized relay address is
   ever invented.
 
-Hunch: wrapper-aware client halves ride the same-channel
-length-prefixed-datagram shape; vanilla clients get UDP ASSOCIATE only
-via the optional local backend. But this is the crate's largest
-unknown — a POC candidate (OQ-SK-07 #2), decided by an ADR before the
-first consumer (the datagram framing is wire-stable once published).
+**Phase model vs alktty-style demux (2026-09-12 discussion).** An
+alternative shape surfaced: alktty's logical demux (input/output/error/
+control sub-streams inside one channel, type byte per chunk, up to the
+u8 = 255 stream-type limit) applied to the SOCKS5 channel — a `data`
+stream and a `udp-relay` stream type would carry the two phases
+structurally. Analysis against RFC 1928's actual structure: **RFC 1928
+is one command per connection, and control and data never interleave in
+either direction.** CONNECT's control conversation ends at the reply,
+after which the channel is a pure byte stream (alktunnels' 0 B
+pass-through conclusion); ASSOCIATE's control stream goes silent after
+the reply — fast-socks5's own `wait_on_tcp` (`src/server.rs:1195`)
+treats any post-reply control byte as protocol garbage
+(`UnexpectedUdpControlGarbage`). So the per-chunk type byte demuxes a
+problem SOCKS5 doesn't have; a **phase model** (raw RFC conversation
+until the reply, then a mode switch synchronized by the reply itself —
+pass-through for CONNECT, `[len: u16 BE]` datagrams for ASSOCIATE)
+subsumes it with no type byte and no sentinel address: wrapper-aware
+clients know "post-reply = datagram mode," the same way they'd know
+"relay stream = datagram mode" under the demux. What the demux genuinely
+buys that the phase model does not: an **additive in-band vocabulary** —
+new stream types extend the wire additively (no format break), whereas
+adding a post-reply phase later is a wire break. alktunnels accepted the
+opposite ("no in-band control path, ever — the escape hatch is a new
+ALPN, a wire change is not"); the demux reintroduces that vocabulary at
+2 B/chunk (type byte + length prefix) on every datagram. The one-way-door
+cost is real either way: demux bakes the vocabulary in before it has a
+user; phase model keeps 0 B overhead on CONNECT and 2 B on ASSOCIATE
+datagrams. Hunch: phase model (simpler, RFC-shaped); the extensibility
+argument is the honest case for the demux if Phase 1 wants the
+vocabulary. Decide via ADR with the first consumer in sight.
+
+Hunch (datagram stage, unchanged): wrapper-aware client halves ride the
+same-channel length-prefixed-datagram shape; vanilla clients get UDP
+ASSOCIATE only via the optional local backend. But this is the crate's
+largest unknown — a POC candidate (OQ-SK-07 #2), decided by an ADR
+before the first consumer (the datagram framing is wire-stable once
+published).
 
 ### OQ-SK-04: fast-socks5 wasm posture (blocks the wasm-clean invariant)
 
@@ -396,6 +428,30 @@ Option A upstream if confirmed. This OQ gates the wasm verification
 command in AGENTS.md (the expected-failure note is already written
 there).
 
+**Fork vs reimplement vs wrap — collapses onto this OQ (2026-09-12
+discussion).** The wrap surface was re-checked and **no structural fork
+is needed** for the channels path: CONNECT rides the interception points
+as-is — `read_command` → own dial → `reply_success` → `transfer` (the
+`examples/router.rs` shape), with the producer's `BiStream` as `T` on
+both sides of `transfer`. UDP is ours either way: `run_udp_proxy_custom`
+takes a custom `transfer` closure for the relay half, `reply_success`
+accepts any `SocketAddr` (the sentinel reply), and
+`new_udp_header`/`parse_udp_request` are public, pure, and reusable.
+The one hardwired piece — `run_udp_proxy`'s peer relay socket
+(`udp_bind_random_port`, socket2-based, called inside the default
+handler before `run_udp_proxy_custom` even runs) — is in the *default*
+handler, not the protocol: the wrapper's custom closure never needs it.
+So the fork-vs-reimplement decision is not about the channels surface
+at all; it is driven by the wasm question above: **wrap (with the
+upstream `net` feature-gate ask) if the gating lands cleanly;
+reimplement if the gating turns out structural.** The reimplementation
+is genuinely small — RFC 1928 over `tokio::io` generics is the
+alktunnels-codec scale of code — and fast-socks5 would remain the
+differential-test oracle for RFC edge cases (reply-code mapping,
+domain addressing, fragmentation) either way. A full fork (divergent
+copy we publish) is the least attractive branch: it duplicates the
+maintenance without buying anything the two options above don't.
+
 ### OQ-SK-05: Target policy and egress scoping
 
 SOCKS5 is arbitrary-egress by nature — the open gate plus the target
@@ -536,11 +592,13 @@ Candidate reading for the research specialist (to be expanded):
       dialer hunch); decide in Phase 1 against the spec
 - [ ] OQ-SK-02 (auth mapping) — posture drafted (identity seam
       primary, in-band auth for the local backend); decide in Phase 1
-- [ ] OQ-SK-03 (UDP ASSOCIATE) — the shape space written; resolve via
-      research + POC #2, ADR before the first consumer
+- [ ] OQ-SK-03 (UDP ASSOCIATE) — the shape space written (including
+      the phase-model-vs-alktty-demux analysis); resolve via research
+      + POC #2, ADR before the first consumer
 - [ ] OQ-SK-04 (fast-socks5 wasm) — verify empirically (POC #4);
-      likely an upstream feature-gating ask; file early per AGENTS.md
-      convention 17
+      the fork-vs-reimplement-vs-wrap decision collapses onto this
+      OQ; likely an upstream feature-gating ask; file early per
+      AGENTS.md convention 17
 - [ ] OQ-SK-05 (target policy) — folded into the OQ-SK-01 decision;
       scope-gate convention pinned in Phase 1
 - [ ] OQ-SK-06 (noq client) — research pass (trait-shape comparison,