docs: align remaining alloc-failure references with ADR-010 §2A (prepublish review for v0.3.0)

- README.md ADR index: ADR-010 status notes the §2A amendment
- ADR-009: amendment note records the §2A supersession (in-band path
  shrinks to nothing from registered producers)
- ADR-010 Consequences: dial_failed replaces the stale in-band
  allocate_failed retry-policy bullet
- AllocFailed doc comment + tty-backend.md: describe both failure
  surfaces (direct-path frame / channels-path dial_failed)

docs/architecture/tty-backend.md

@@ -125,8 +125,10 @@ pub enum TtyError {
 
 - `AllocFailed` — the PTY couldn't be allocated, the docker exec failed
   to start, the SSH channel request was rejected. Returned by
-  `allocate()`; the adapter sends `{"error":"allocate_failed",...}` and
-  closes (tty-adapter.md §"Negotiation errors").
+  `allocate()`; on the direct path the adapter sends
+  `{"error":"allocate_failed",...}` and closes (tty-adapter.md
+  §"Negotiation errors"); on the channels path the establisher maps it
+  to `channel:open_failed` / `dial_failed` (ADR-010 §2A).
 - `WaitFailed` — the backend couldn't reap the child / determine the
   exit code. Returned by the `exit_code` future; the adapter sends
   `{"type":"exit","code":-1}` (ADR-004 §4).