Issue #10 Task 1: a rejected call's reason had nowhere to live. The
field goes on the base class because these errors all come from one HTTP
response - which class it is and what the peer said are orthogonal.
A fail-closed limiter or breaker backend means the scope cannot emit a
single request, which is exactly scope-level unavailability. But the error
sat directly under PolyGatewayError, so a caller writing only
`except GatewayUnavailableError` dropped it into the catch-all branch:
Redis blips once and a backlog of tasks burns its business failure budget
into the dead letter queue, over a fault a restart would clear.
Three gate paths leak to callers rather than being absorbed by
_record_quietly (try_acquire, try_enter, progress_age_s); each is now
pinned by a test, since none of them had one before.
The two unknown-source sites move to SourceNotConfiguredError instead of
following along. They report a misconfigured source name, not an outage,
and letting them into the retryable family would be the mirror of the bug
being fixed here: the task would retry forever and never surface.
Pure addition ahead of the reparenting, so this commit leaves every
existing caller and test untouched.
SourceNotConfiguredError deliberately stays outside GatewayUnavailableError:
a source name that is not in the limiter's config dict is an assembly
defect, not a transient outage, and folding it into the retryable family
would let a typo retry forever without ever reaching a dead letter queue.
The retry_after_s default is 5.0 rather than 0 because a backlog released
at zero delay would stampede a backend that is already down.
Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.
iomgaa