params: the error-schema declaration + the wire hygiene residue (review-002 D-3, N-3, N-9)

- D-3: socks5_open_spec declares the channel:open_failed
  ErrorDefinition (alktty-ported schema, adapted) — reason enum is
  the full ADR-049 §3 five-code vocabulary matching
  SocksEstablishmentReason's parse table; description carries this
  crate's gate classes (FORBIDDEN / INVALID_INPUT /
  channel:too_many_channels) + the no-phantom-channel property.
  Pinned by open_spec_declares_the_open_failed_error_definition,
  which round-trips every enum value through the typed vocabulary —
  schema and typed view cannot drift. Module doc updated from claim
  to fact.
- N-3: DatagramReader::feed's structurally-unreachable let-else
  restructured — the header read folds into the let-else's else arm
  (live-else shape); zero behavior change, wire suite green
  unchanged.
- N-9: is_mid_frame kept; doc names the anticipated batch-2 consumer
  (the front-door bridges) so dead-surface sweeps don't flag it.

Verification:
- cargo test: 357 passed, 0 failed (all suites)
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean
- cargo check --target wasm32-unknown-unknown: clean
- cargo clippy --target wasm32-unknown-unknown -- -D warnings: clean

Task: tasks/implementation/fix-hygiene-residue.md (completed)

tasks/implementation/fix-hygiene-residue.md

@@ -1,7 +1,7 @@
 ---
 id: implementation/fix-hygiene-residue
 name: "Fix — error-schema declaration + small hygiene residue (review-002 D-3, N-3, N-9)"
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: low
@@ -57,16 +57,16 @@ postures (`docs/reviews/002-protocol-core-implementation-review.md`):
 
 ## Acceptance Criteria
 
-- [ ] `socks5_open_spec` declares the `channel:open_failed`
+- [x] `socks5_open_spec` declares the `channel:open_failed`
       `ErrorDefinition` (the alktty-ported schema, adapted) — or the
       doc is reworded to the honest empty posture with the reason
       recorded
-- [ ] A test pins the declaration (mirroring alktty's) — or the
+- [x] A test pins the declaration (mirroring alktty's) — or the
       reworded doc matches the spec exactly
-- [ ] The `feed` let-else is restructured; zero behavior change
+- [x] The `feed` let-else is restructured; zero behavior change
       (the wire suite green unchanged)
-- [ ] `is_mid_frame`'s doc names the anticipated consumer
-- [ ] `cargo test` green; clippy/fmt clean; both wasm checks clean;
+- [x] `is_mid_frame`'s doc names the anticipated consumer
+- [x] `cargo test` green; clippy/fmt clean; both wasm checks clean;
       `cargo doc --no-deps` clean
 
 ## References
@@ -82,8 +82,82 @@ postures (`docs/reviews/002-protocol-core-implementation-review.md`):
 
 ## Notes
 
-> Agent fills this during implementation.
+- **D-3 — declared, not reworded.** The review's lean (the declare
+  side) worked with no substrate constraint: the registry accepted
+  the definition on the first try. The schema is alktty's
+  `channel:open_failed` block
+  (`/workspace/@alkdev/alktty/src/channels.rs:165-208`), adapted:
+  the `reason` enum carries all five ADR-049 §3 codes
+  (`dial_failed` / `unknown_resource` / `resource_shortage` /
+  `handler_error` / `timeout`) — alktty's enum has four because its
+  establisher has no `resource_shortage` path, while this crate's
+  declared vocabulary is chosen to match
+  `SocksEstablishmentReason`'s parse table exactly (the typed view a
+  consumer branches on), not the current default establisher's
+  reachable subset (`socks5_session_establisher` is a session gate
+  returning `Establishment::default()`; a composed/registry
+  establisher could produce any of the five). The `description`
+  text is rewritten to this crate's establishment story: the
+  registry gate classes first (`FORBIDDEN` / `INVALID_INPUT` /
+  `channel:too_many_channels` — the ACL runs before the
+  establisher, ADR-006), then the establishment vocabulary, then
+  the no-phantom-channel property. One judgment call worth noting
+  for the review round: the declared enum is *wider* than the
+  default producer's behavioral surface (which produces none of the
+  five today — the dial is in-stream, post-open). That is the
+  correct direction to over-declare: `error_schemas` discloses the
+  op's contract shape (any registration of this op, per ADR-049
+  §4), and the enum is the wire's parse table
+  (`SocksEstablishmentReason::parse`), so a stricter enum would
+  under-declare for custom-establisher registrations.
+- **The pinning test cross-checks schema vs typed vocabulary.**
+  Beyond mirroring alktty's declaration pin
+  (`:780-807`), `open_spec_declares_the_open_failed_error_definition`
+  asserts each enum value round-trips through
+  `SocksEstablishmentReason::parse`/`as_str` — the declared schema
+  and the typed vocabulary cannot drift silently (the exact D-3
+  drift class, pinned shut).
+- **N-3 — restructured to a live-else shape, not the review's
+  sketch.** Both of the review's alternates were considered: the
+  `let Some(&mut (declared, ref mut acc))` match has the same
+  dead-arm problem (it must fire *after* the header read, which is
+  exactly the unreachable region), and the re-order-to-drop-else
+  sketch inverts the loop's exit order. The landed shape folds the
+  header read into the let-else's else arm
+  (`let Some((declared, acc)) = self.current.as_mut() else {
+  <read header; continue; }`) — every branch is live, the
+  "current set above" invariant is structural (the else arm is the
+  only place `current` is set), and the body is untouched. Zero
+  behavior change: the full wire suite passes unmodified.
+- **N-9 — kept with the note.** One doc paragraph naming the
+  anticipated batch-2 consumer (a front-door bridge driving its own
+  reader loop — the doors) and citing the review finding.
 
 ## Summary
 
-> Agent fills this on completion.
+All three residue items closed in one pass (`src/params.rs`,
+`src/wire.rs`):
+
+1. **D-3**: `socks5_open_spec` now declares the
+   `channel:open_failed` `ErrorDefinition` — alktty-ported schema
+   (`{ reason (enum of the five ADR-049 §3 codes), message }`,
+   both required), description adapted to this crate's gate classes
+   + establishment story. Module doc updated from claim to fact.
+   Pinned by `open_spec_declares_the_open_failed_error_definition`
+   (declaration + enum round-trips through
+   `SocksEstablishmentReason` — schema and typed vocabulary pinned
+   drift-free).
+2. **N-3**: `DatagramReader::feed`'s structurally-unreachable
+   let-else restructured into a live-else shape (header read in the
+   else arm); zero behavior change — the wire suite green unchanged.
+3. **N-9**: `is_mid_frame` kept; its doc names the anticipated
+   consumer (the batch-2 front-door bridges) so future dead-surface
+   sweeps don't flag it.
+
+Verification: `cargo test` 357 passed / 0 failed (all suites);
+`cargo clippy --all-targets -- -D warnings` clean; `cargo fmt
+--check` clean; `cargo doc --no-deps` clean; `cargo check
+--target wasm32-unknown-unknown` clean; `cargo clippy --target
+wasm32-unknown-unknown -- -D warnings` clean. Doc-vs-code drift is
+gone either way (review-002 D-3 closed; the review-round gate item
+"the `ErrorDefinition` is declared" holds).