* ci: neutralise time.sleep in test code for coverage runs

Sprint 0 (coverage). Coverage is now reported correctly via
pytest-cov + xdist, but a handful of TestCircuitBreaker tests
use bare time.sleep(1.1) to wait out the 1.0s recovery_timeout.
That was a 3.3-second tax per worker on every xdist run, and
the suite could not be collected on Windows in a reasonable
time without the cap. The conftest autouse fixture
_fast_sleep caps test sleeps at 1ms, which is well above
the cancellable-wait regression threshold (0.05s) and zero
impact on retries (the existing per-test monkeypatch covers
the time.monotonic path).

Three TestCircuitBreaker tests now advance the wall clock
via _advance_clock(monkeypatch) instead of sleeping, so the
recovery transition fires deterministically.

Opt-out: @pytest.mark.slow_sleep on a test class keeps
the real wall clock (e.g. test_ping_chain_emits_heartbeats_on_time_schedule
needs real-time progression for the scheduler thread).

Verified: 1237 passed, 7 skipped, 29 warnings in 34.92s;
combined coverage 80.98% (vs 79.26% on master 29caae9).

* chore(release): 0.13.12 — CI / coverage-testability

Bump SDK 0.13.11 -> 0.13.12. CI scope only — no on-wire change,
no SDK_MIN_VERSION bump, no public API change. Backends on 1.0.0
keep working unchanged.

Pyproject version + __version__ + CHANGELOG entry. The
mechanical work (conftest autouse _fast_sleep, _advance_clock
helper, slow_sleep marker, codecov pytest-cov config) shipped
in commit e6dd730; this commit just re-tags that work as
0.13.12 so the published wheel exposes the new version string.

Verified: 1237 passed, 7 skipped, 29 warnings; combined coverage
80.87% (vs master 29caae9 79.26% via Codecov API; the 0%
in the README badge was the coordinator-only coverage bug
Sprint 0 already fixed in PR #70).