feat: channels-path establisher migration (alkcall 0.5.0 / ADR-049) — bump to 0.2.0

Adopt alkcall 0.5.0's channel-open establishment phase (ADR-049 —
review 006 E-01 + N-1) and migrate the channels-path semantic failures
per its §5 sequencing (alktty ADR-010).

- `register_openable` registers `channels/tty/sub` with an establisher
  (`register_openable_with_establisher`): full `NegotiateRequest`
  parse of schema-valid `input`, `carriage == "raw"`, non-empty `cmd`,
  backend lookup, and the ADR-050 ownership check run before the open
  reply; rejections are `channel:open_failed` with `details.reason`
  (`unknown_resource` / `handler_error` / `timeout`) — no phantom
  channel (the SSH contract holds consumer-visibly)
- `backend.allocate` deliberately stays in the pump handler:
  `Establishment` is payloadless so the `TtyHandle` cannot cross the
  establisher→handler boundary, and re-allocating would violate
  ADR-005's kill-on-Drop contract — `allocate_failed` remains the one
  in-band failure class on the channels path (pinned by test)
- `TtySessionError::ChannelsOpen` carries alkcall's typed
  `ChannelOpenError` (`#[from]`) instead of a flattened `String` —
  the N-1 fix at alktty's layer (breaking)
- channels-path semantic failures change shape from
  `NegotiationRejected` in-band frames to `channel:open_failed` call
  errors (breaking); the direct-ALPN path is unchanged
- `tty_open_spec()` gains a `description` (review 006 E-02) and an
  ErrorDefinition for `channel:open_failed` (ADR-016 — disclosed via
  services/schema)
- alkcall = "0.5.0"; version 0.2.0; ADR-010 + ADR-009 amendment +
  tty-adapter.md + CHANGELOG

Verification: cargo test (112 lib + integration), cargo test
--all-features (136), clippy --all-targets -D warnings (host + wasm),
fmt --check, cargo doc --no-deps clean; wasm32-unknown-unknown check
confirms the default crate stays wasm-clean.

docs/architecture/README.md

@@ -19,7 +19,8 @@ Syntax Tree) document for the wire format, and the ADRs.
 ## Applicable ADRs
 
 Ported from the alknet mono-repo and renumbered into alktty's ADR
-range (001..009; ADR-009 is alktty-native). The alknet originals at
+range (001..009; ADR-009 and ADR-010 are alktty-native). The alknet
+originals at
 `/workspace/@alkdev/alknet/docs/architecture/decisions/` remain the
 authoritative source for any ADR not yet ported, and for the alknet
 ADRs referenced by alknet number in the docs below (which are not
@@ -35,7 +36,8 @@ tty-specific and therefore not ported into alktty's ADR range).
 | [006](decisions/006-negotiation-framing-self-contained.md) | Self-Contained Negotiation Framing (No alkcall-Internal-Wire-Types Dependency) | alknet ADR-057 | Accepted |
 | [007](decisions/007-tty-inside-channels.md) | TTY Inside Channels — Sub-Streams, Not Wire Format | alknet ADR-077 | Accepted (**reversed by ADR-008** — kept for historical context) |
 | [008](decisions/008-channels-pure-channel-multiplexing.md) | Channels Pure Channel Multiplexing (8-Byte Header, No `stream_type`) | alknet ADR-093 | Accepted (amends alknet ADR-071/074; reverses ADR-007) |
-| [009](decisions/009-channels-open-op-is-the-negotiation.md) | The Channels Open Op's `input` Is the Negotiation | alktty-native | Accepted (resolves review #001 L1; amended by review #002 R4 — parse failure is a client-visible error frame) |
+| [009](decisions/009-channels-open-op-is-the-negotiation.md) | The Channels Open Op's `input` Is the Negotiation | alktty-native | Accepted (resolves review #001 L1; amended by review #002 R4 — parse failure is a client-visible error frame; amended by ADR-010 — semantic failures move into the establisher) |
+| [010](decisions/010-channels-establisher-migration.md) | Channels-Path Establishment Failures Move into an Establisher | alktty-native | Accepted (adopts alkcall 0.5.0 ADR-049; amends ADR-009's R4 amendment — in-band frames shrink to `allocate_failed`) |
 
 ## Key Design Principles