params/error: the wire-stable open-op surface + the typed open-error vocabulary

- params: the ADR-001/ADR-006 constants (`channels/socks5/sub`,
  `alk/socks5`, `socks5:open`, `socks5:bind`), `socks5_open_spec()`
  (Sub/External/channel-open marker; trivial empty-object input schema
  with loud `additionalProperties: false`; reply
  `{channel_id: integer, minimum 1}` required; `socks5:open` scope),
  `Socks5Params {}` (deny_unknown_fields — empty stays valid forever),
  the `establishment_reason` helper re-export.
- error: `SocksOpenError` (the ADR-049 §4 typed surface, shaped on
  alktunnels' `TunnelOpenError`), the typed
  `SocksEstablishmentReason` vocabulary (the five reason codes,
  parse/as_str; unknown codes never aliased), `SocksOpenError::reason()`
  as the typed view, and `SocksSessionError` (open vs in-stream `Rfc`
  vs `Io` — a refused target is an RFC reply code through the session
  type, consumer.md §Error surface). `SocksError`/`SocksServerError`/
  `ReplyError` re-exported from the forked core.
- lib.rs re-exports the new public surface (convention 15).
- 9 new tests: spec shape, constants, serde round-trip +
  unknown-field rejection, the reason mapping table, the
  non-establishment classes (`FORBIDDEN`/`INVALID_INPUT`/
  `too_many_channels`), future-reason non-aliasing, the three-plane
  session surface.
- task `implementation/core-params-errors`: status → completed;
  Notes + Summary filled.

Verification: cargo test (104 green), clippy --all-targets -D
warnings clean, fmt --check clean, doc --no-deps zero warnings,
both wasm checks (check + clippy on wasm32-unknown-unknown) clean.

tasks/implementation/core-params-errors.md

@@ -1,7 +1,7 @@
 ---
 id: implementation/core-params-errors
 name: Core — params schema + scope constants + typed error surface
-status: pending
+status: completed
 depends_on: [implementation/core-crate-deps]
 scope: narrow
 risk: low
@@ -81,8 +81,71 @@ error-reason mapping table.
 
 ## Notes
 
-> To be filled by implementation agent
+> Filled by the implementation agent (2026-09-23).
+
+- `src/params.rs`: the four wire-stable constants (`OP_SOCKS5_OPEN`,
+  `SOCKS5_ALPN`, `SOCKS5_OPEN_SCOPE`, `SOCKS5_BIND_SCOPE`), the
+  trivial-params struct `Socks5Params {}` (serde
+  `deny_unknown_fields`, + `Serialize` so tests round-trip it;
+  `as_json()` names the canonical wire form), `socks5_open_spec()`
+  (Sub/External/channel-open marker on `alk/socks5`; input schema
+  `{type: object, additionalProperties: false}`; reply schema
+  `channel_id` integer minimum 1 required; `required_scopes =
+  ["socks5:open"]`), and the `establishment_reason` helper re-export
+  (the alktunnels posture — delegates to
+  `ChannelOpenError::establishment_reason`).
+- `src/error.rs`: `SocksOpenError` (single `Open` variant wrapping
+  `ChannelOpenError` — the same shape as alktunnels'
+  `TunnelOpenError`; the `From` impl covers the
+  `channel:open_failed`/`FORBIDDEN`/`INVALID_INPUT` classes), the
+  typed `SocksEstablishmentReason` vocabulary (ADR-049 §3's five
+  codes, `parse`/`as_str`; unknown codes are `None` — never silently
+  aliased, the vocabulary is one-way), `SocksOpenError::reason()` as
+  the typed view, and `SocksSessionError` (open errors vs in-stream
+  `Rfc(SocksError)` vs `Io`; the RFC reply-code refusals ride the
+  session type per consumer.md §Error surface). `SocksError`/
+  `SocksServerError`/`ReplyError` re-exported from `rfc1928`
+  (`SocksServerError` lives in `rfc1928::server`, not the module
+  root).
+- lib.rs re-exports both modules' public surface (convention 15).
+- Deliberate deviations from the task text: the params struct
+  derives `Serialize` (test round-trip needs it; harmless on the
+  wire — the consumer sends `json!({})`); `SocksEstablishmentReason`
+  is a separate named type rather than an inline match so the
+  mapping table is testable in both directions; unknown reason codes
+  do NOT alias a variant (one-way vocabulary discipline).
+- rustdoc links to `error::SocksOpenError` from `params` are
+  fully-qualified (`crate::error::…`) — plain `[SocksOpenError]`
+  does not resolve cross-module in this crate's layout.
 
 ## Summary
 
-> To be filled on completion
+> Filled on completion (2026-09-23).
+
+Implemented the wire-stable open-op surface and the typed
+open-error vocabulary:
+
+- `params`: constants + `socks5_open_spec()` per ADR-001/ADR-006
+  exactly (op id `channels/socks5/sub`, ALPN `alk/socks5`, scopes
+  `socks5:open`/`socks5:bind`, trivial empty-object input schema with
+  loud `additionalProperties: false`, reply
+  `{channel_id: integer, minimum 1}` required).
+- `Socks5Params {}` with `deny_unknown_fields` — empty object valid,
+  unknown fields rejected.
+- `error`: `SocksOpenError` mapping the ADR-049 §4 reasons
+  (`dial_failed`/`unknown_resource`/`resource_shortage`/
+  `handler_error`/`timeout`) through the typed
+  `SocksEstablishmentReason` (unknown codes never aliased); the
+  registry's `FORBIDDEN`/`INVALID_INPUT` classes correctly
+  non-establishment; `SocksSessionError` shaped open-vs-Rfc-vs-Io
+  per consumer.md §Error surface.
+- 9 new tests (spec shape, constants, serde round-trip + unknown-field
+  rejection, the reason mapping table, non-establishment classes,
+  future-reason non-aliasing, the three-plane session surface).
+- lib.rs re-exports the public surface.
+
+Verification: `cargo test` (104 green: 67 lib + 37 rfc1928 + 0
+doc), `cargo clippy --all-targets -- -D warnings` clean,
+`cargo fmt --check` clean, `cargo doc --no-deps` zero warnings, both
+wasm checks (`cargo check` + `cargo clippy` on
+`wasm32-unknown-unknown`) clean.