feat: allocation moves into the channels establisher (alkcall 0.6.0 / review 007 R-01) — bump to 0.3.0

- Bump alkcall 0.5.0 -> 0.6.0 (Establishment plan payload, ADR-049
  amendment 2); alktty 0.2.0 -> 0.3.0 (breaking channels-path
  allocation-failure shape)
- make_tty_establisher runs backend.allocate; failure maps to
  EstablishmentError::DialFailed (channel:open_failed reason
  dial_failed); the TtyHandle crosses to the pump handler via a
  private per-open AllocatedHandle one-shot slot (the handle is not
  Sync — it cannot be the ChannelPlan directly)
- make_tty_open_handler gains the Option<ChannelPlan> parameter; a
  plan drives the new drive_session_pre_allocated (pumps only), a
  None plan falls back to drive_session_pre_negotiated (inline
  validate-and-allocate — defense-in-depth for no-establisher
  registrations)
- tty_open_spec's channel:open_failed ErrorDefinition declares
  dial_failed (four reachable reasons)
- Pinned in-band allocate test flipped:
  allocate_failure_fails_open_as_dial_failed (end-to-end, no channel
  survives); establisher unit gate for the plan slot + failure
  mapping
- ADR-010 amended (§2A; §2 kept as historical record), tty-adapter.md,
  session/channels/adapter docs, CHANGELOG, AGENTS.md alkcall pin
  updated

Verification: cargo test (113) + --all-features (137), clippy
-D warnings (native + wasm32, default + all-features), fmt, doc,
wasm32 check — all clean

docs/architecture/tty-adapter.md

@@ -177,17 +177,18 @@ entering raw mode. The error response shape:
 On the **direct-ALPN path** this is the failure surface for every
 negotiation/allocation failure class (below). On the **channels path**
 the semantic classes (unknown backend, malformed negotiation,
-ownership denial) are rejected by the establisher before the open
-reply and surface as `channel:open_failed` call errors (alkcall 0.5.0
-ADR-049 / alktty ADR-010) — the in-band frames on that path are
-defense-in-depth arms plus the one non-migratable class
-(`allocate_failed`, ADR-010 §2).
+ownership denial) **and allocation** are rejected by the establisher
+before the open reply and surface as `channel:open_failed` call
+errors (alkcall ADR-049 / alktty ADR-010, as amended for alkcall 0.6's
+`Establishment` plan payload — ADR-010 §2A) — the in-band frames on
+that path are defense-in-depth arms only (reachable from
+no-establisher registrations).
 
 | Error | When | Shape |
 |-------|------|------|
 | `unknown_backend` | the `backend` string is not in the adapter's backend map — direct path, or the channels path's defense-in-depth arm (the establisher rejects it as `channel:open_failed` / `unknown_resource` first) | `{"error":"unknown_backend","backend":"..."}` |
 | `malformed_negotiation` | the negotiation frame failed to parse as JSON or failed `NegotiateRequest` validation — on the direct path the wire frame; on the channels path the open op's `input` (schema-valid values can still fail the typed parse, e.g. `cwd` typed as a number, because the schema is deliberately partial — the establisher rejects it as `channel:open_failed` / `handler_error` first; the handler-side frame is defense-in-depth, ADR-010 §2) | `{"error":"malformed_negotiation","message":"..."}` |
-| `allocate_failed` | `backend.allocate()` returned a `TtyError` — on both paths: the establisher cannot carry the allocated handle across to the pump handler (ADR-010 §2), so allocation failure is the one in-band failure class on the channels path | `{"error":"allocate_failed","message":"..."}` |
+| `allocate_failed` | `backend.allocate()` returned a `TtyError` — the failure surface of the **direct path**. On the channels path the establisher runs `allocate` (the handle crosses to the pump handler via alkcall 0.6's `Establishment` plan payload — ADR-010 §2A) and its failure is a `channel:open_failed` call error (`details.reason == "dial_failed"`) | `{"error":"allocate_failed","message":"..."}` |
 
 After sending the error response, the adapter closes the write half of
 the bidi stream. The client reads the error frame and treats stream close
@@ -369,7 +370,7 @@ architectural commitment.
 | Backend cleanup on session cancel | [ADR-005](decisions/005-backend-cleanup-on-session-cancel.md) | Dropping `exit_code` future kills the session target; the adapter triggers it by dropping the `TtyHandle` on cancel |
 | Channels pure channel multiplexing | [ADR-008](decisions/008-channels-pure-channel-multiplexing.md) | The same session driver runs in both direct and channels modes; only the `BiStream` source differs |
 | Negotiation carried in the open op | [ADR-009](decisions/009-channels-open-op-is-the-negotiation.md) | The channels path carries no second negotiation frame; the open op's registry-validated `input` is the negotiation (`drive_session_pre_negotiated`) |
-| Channels establishment failures are call errors | [ADR-010](decisions/010-channels-establisher-migration.md) | The channels path's semantic failures are rejected by the establisher (`channel:open_failed`, alkcall ADR-049); `allocate_failed` stays in-band |
+| Channels establishment failures are call errors | [ADR-010](decisions/010-channels-establisher-migration.md) | The channels path's semantic failures and allocation are rejected/run in the establisher (`channel:open_failed`, alkcall ADR-049; alkcall 0.6 plan payload carries the `TtyHandle`); no in-band frame from a registered producer |
 | Dynamic resource ownership | alknet ADR-050 | Terminal sessions as runtime-spawned resources; the adapter's access-control shape |
 
 ## Open Questions
@@ -394,8 +395,9 @@ architectural commitment.
   registry-validated `input` is the negotiation)
 - [ADR-010](decisions/010-channels-establisher-migration.md) — the
   channels path's semantic failures are establisher call errors
-  (`channel:open_failed`); the in-band frames shrink to
-  `allocate_failed`
+  (`channel:open_failed`); since alkcall 0.6 the establisher also
+  allocates (the `TtyHandle` crosses via the plan payload), so no
+  failure class arrives in-band from a registered producer
 - alknet ADR-050 — the ownership model the adapter's access control
   declares against
 - alknet ADR-007 — `Connection`, `accept_bi`, the handler-receives-