feat: map Worldtree b1 eager turn-launch statuses (409/503) in stream_turn

Worldtree v1.0.0b1 (#331) decoupled turn execution from the SSE connection,
so turn-launch failures now arrive EAGERLY as an HTTP status before any
stream: 409 agent_not_available (pre-b1 was a 200 + in-stream error event)
and 503 (retryable turn-launch / infra failure). stream_turn previously
funneled both into a generic SseConnectFailed.

Map them to typed SseConnectFailed subclasses — AgentNotAvailable (409) and
TurnLaunchUnavailable (503, retryable=True) — carrying the parsed
error_code/message from the {detail:{error_code,message}} envelope.
Subclassing keeps existing `except SseConnectFailed` handlers working with
zero changes (POST-003 preserved — no synthetic event yielded; raise mirrors
reconnect_turn's 400/410/412 pattern).

worldtree-dev confirmed 409/503 are real runtime statuses; the OpenAPI 2.1.0
gap (not enumerating them) is theirs to fix (doc-completeness, not a wire
break). The 503 error_code is being re-pinned upstream (today internal_error
-> likely not_ready); our handling keys on STATUS so it's robust to the final
code — tighten the 503 default once they confirm.

Body shape live-confirmed against demo b1's 404/401 responses. Suite 509 green.
Contract docs/contracts/issues/1.contract.md updated.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.18.2"
+version = "0.18.3"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"