test(reforcexy): add the coverage gate and a shared QA harness

test(reforcexy): add the coverage gate and a shared QA harness

Brings ReforceXY to gate parity with QuickAdapter: a `.coveragerc`, a
coverage pin in the QA image, and the three modules that make the gate
auditable — a shared harness, a suite contract and a floor guard.

qa_support.py owns the mutable process globals and the fixture builders,
and installs the test-order shuffle. The suite contract pins import
identity: every measured file under exactly one dotted name, every test
module under its bare stem. That second guard is what closes the live
defect this change fixes — `test_training_observations.py` imported
`ReforceXY.tests.test_review_contracts`, registering a second module object
for a file discovery had already registered, so every reset in the harness
would have cleaned a module the code under test never touches. The three
existing modules are renamed to the file-derived `test_model_*` convention
and reparented onto the shared base.

The floor guard replaces QuickAdapter's text scan of coverage's default
exclusions with a differential parse using coverage's own parser, because
the scan over-approximates here (three `...` lines inside a docstring) and
is blind to the branch-level family, which removes 27% of this denominator
with no config key at all.

Measured 57.1% against a floor of 57. The agreed 70% target is not met:
`MINIMUM_FLOOR` is pinned to what was actually reached, not to the target,
because a floor the suite cannot clear is a control the project believes it
has and does not. The gap and the remaining rows are in the pull request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>