rfc1928: the targeted fork — rename + convention pass (ADR-013)

The mechanical half of the posture change: src/vendor/ ->
src/rfc1928/ (module alksocks::rfc1928), tests/vendor-differential.rs
-> tests/rfc1928-differential.rs (git mv — history preserved).

- Fork point pinned: the tree as of 5b18d4d (the extraction review's
  gate commit); provenance permanent in the module docs and the
  extended LICENSE notice (upstream MIT preserved, fork-point +
  post-fork modification statement added — extend, never replace)
- The lint-allow posture dissolved: all module-level #[allow]s
  removed and the suppressed smells fixed (u8::max_value() ->
  u8::MAX, needless lifetimes elided, match_like_matches_macro
  applied, io::Error::new(Other) -> io::Error::other, the
  AuthMethod::new self-taking signature renamed
  into_starting_state, the F-4 ToSocketAddrs string re-worded — it
  no longer names the cut resolve_dns(), the unused ready! macro
  and dead AuthenticationMethod::from_u8 removed, the upstream
  commented-out async-std note folded into the impl doc)
- Module docs rewritten: ownership statement (project conventions
  apply unconditionally), extraction provenance (ADR-011) +
  ownership (ADR-013) + maintenance posture (behavioral reference,
  hand-port path)
- Differential suite survives the rename untouched (32 tests pin
  bytes, not upstream text)

Verification: cargo test (38+32 green), cargo clippy --all-targets
-D warnings, cargo fmt --check, cargo check/clippy
--target wasm32-unknown-unknown, cargo test --all-features, cargo
doc --no-deps — all pass.

docs/architecture/decisions/013-targeted-fork.md

@@ -96,7 +96,7 @@ vendor.md, and the review task. "The tree as of the review commit
   fork-point statement + the modification note.
 - **The reference checkout survives as a behavioral reference.**
   `/workspace/fast-socks5` stays for differential testing of RFC
-  edge cases; the differential suite (tests/vendor-differential.rs)
+  edge cases; the differential suite (tests/rfc1928-differential.rs)
   pins bytes, not upstream text, so it survives the rename
   untouched. Textual-diff discipline is retired; behavioral
   differential testing is retained.