feat: add an optional per-call wall-clock deadline

Give one logical call an optional hard wall-clock boundary (issue #22).
Leaving it unset keeps 1.3.5 behaviour verbatim: the timeout context is
never entered when deadline_s is None.

- new deadline.py: ensure_call_deadline() range check (None or a finite
  positive number; bool/0/nan/inf and out-of-range ints are rejected as
  ValueError so OverflowError never leaks) plus with_call_deadline(),
  which distinguishes an expiry from a TimeoutError raised by the body
  or its cleanup via a local-variable identity comparison rather than
  cm.expired() alone
- new CallDeadlineExceeded: deliberately outside the four categories and
  not a GatewayUnavailableError, and carries no retry_after_s
- new {SCOPE}__CALL_DEADLINE_S key, guarded on the env, direct
  construction and dataclasses.replace paths
- three clients take a call_deadline_s constructor argument and a
  keyword-only per-call override on chat/embed/recognize_text/
  parse_layout; None inherits the assembled value
- validation runs before the awaitable is created, so an illegal value
  cannot strand an un-awaited coroutine
- one embed call shares a single deadline across all of its batches
- import-linter gains a polygateway.deadline layer

- cover where the deadline lands: backoff sleep, admission polling,
  the structured re-ask ladder and embedding's batch loop, plus the
  empty-texts early return that stays outside it
- cover what an expiry costs: exactly one terminal_failure row carrying
  error_type=CallDeadlineExceeded, a cancelled attempt row sharing its
  logical_call_id, cleanup that outlives the deadline (lower bound only)
  and an already-billed success being discarded
- pin the injected clock as orthogonal: a 10^6 second jump never expires
  a call, yet total_latency_ms still reads that clock

src/polygateway/__init__.py

@@ -10,6 +10,7 @@ from polygateway.config import EmbeddingSettings, GatewaySettings, OcrSettings
 from polygateway.embedding import EmbeddingClient
 from polygateway.errors import (
     AllSourcesExhausted,
+    CallDeadlineExceeded,
     CircuitOpenError,
     GatewayUnavailableError,
     GovernanceBackendError,
@@ -59,6 +60,7 @@ __all__ = [
     "Effort",
     "AllSourcesExhausted",
     "CallStats",
+    "CallDeadlineExceeded",
     "CircuitOpenError",
     "EmbeddingClient",
     "EmbeddingResponse",