docs: the targeted fork — ADR-013, the RFC 1928 core is owned

ADR-013 supersedes ADR-011's maintenance posture (not its mechanics):
the extracted T-generic subset is fully owned code, the vendor
framing is retired. Module renamed src/vendor/ -> src/rfc1928/
(naming decided in the ADR: 'vendor' names an ownership relation
now gone; 'socks5' would stutter and collide with the future
producer/consumer layers). Fork point = the tree as of the
extraction-review commit (5b18d4d); provenance stays permanent
(module docs + the extended LICENSE notice).

- ADR-013: fork posture, license mechanics (notice extended, not
  replaced; post-fork modifications under the crate license),
  hand-port replaces re-vendor, lint-allow posture dissolved
- ADR-011 marked Superseded (extraction mechanics remain
  authoritative); ADR-002 references re-pointed
- vendor.md -> rfc1928.md rewritten as the owned-core spec (the cut
  table re-read as the hand-port map)
- spec sweep: README index (ADR-013 row, doc table, principle 8),
  overview (module map, deps, ADR table), producer/consumer/wire/
  bast reference vocabulary, open-questions OQ-SK-04 resolution,
  AGENTS.md conventions 4/17 + verification note + context bullet,
  Cargo.toml description

Verification: cargo doc --no-deps clean; no stale vendor.md/src-vendor
references outside the ADR-011 historical text.

docs/architecture/bast.md

@@ -18,7 +18,7 @@ conforming to the BAST meta-schema at `https://alk.dev/bast/v1/schema`
 well-formedness.
 
 **alksocks does not depend on alktype.** The runtime codec is the
-vendored fast-socks5 address encoding + the length-framing codec
+forked RFC 1928 address encoding + the length-framing codec
 (shaped on alktunnels' `frame_datagram`/`DatagramReader`, ADR-003);
 this BAST document is the human-readable contract describing what
 those round-trip. If runtime validation against the BAST becomes
@@ -37,8 +37,8 @@ plus raw pass-through.
 Per the BAST format spec, a BAST document describes **binary data
 layouts** — not JSON payloads. The RFC 1928 conversation itself
 (negotiation, command, replies, the RFC UDP datagram header at front
-doors) is specified by RFC 1928 and implemented by the vendored
-fast-socks5 codecs ([vendor.md](vendor.md)) — it is not re-specified
+doors) is specified by RFC 1928 and implemented by the forked
+RFC 1928 codecs ([rfc1928.md](rfc1928.md)) — it is not re-specified
 here. This document covers exactly the alksocks-specific binary
 framing:
 
@@ -51,7 +51,7 @@ What this BAST deliberately does **not** cover:
 - **The RFC 1928 wire rules** — method negotiation, command/reply
   encoding, the `addr` sub-layout (ATYP + addr + port), the RFC UDP
   datagram header (RSV/FRAG/ATYP/addr/port, front doors only). The
-  RFC is normative; the vendored codecs implement it.
+  RFC is normative; the forked codecs implement it.
 - **The open-op params JSON shape** (`{}`) — UTF-8 JSON on the call
   plane; ADR-001.
 - **The channels 8-byte chunk header** — the channels layer's own
@@ -99,7 +99,7 @@ arithmetic).)
 
 Maps 1:1 to the on-wire bytes: `len` as `uint16` big-endian (2
 bytes), then exactly `len` bytes = `addr + data`. The `addr`
-sub-layout is the RFC 1928 address encoding as vendored
+sub-layout is the RFC 1928 address encoding as forked
 (`TargetAddr::to_be_bytes()`: ATYP `0x01` IPv4 / `0x03` domain /
 `0x04` IPv6, then the address, then the u16 BE port). RSV/FRAG do
 not ride the tunnel wire (ADR-003); FRAG ≠ 0 is refused at front
@@ -129,7 +129,7 @@ upstream's, not re-specified here.
 - [ADR-003](decisions/003-udp-handle-swap.md) (the handle swap + the
   frame), [ADR-001](decisions/001-alpn-and-open-op.md) (the JSON
   open-op shape — outside BAST scope),
-  [ADR-011](decisions/011-inline-vendor.md) (the vendored codecs)
+  [ADR-011](decisions/011-inline-vendor.md) (the forked codecs)
 - [wire.md](wire.md) (the normative prose; §The Data Plane)
 - alktunnels `bast.md` (the BAST precedent — same meta-schema, same
   "binary framing only" scope rule; the length codec's original