Opus gate finding (BLOCKING): the two #4385 archive-reappears regression tests
mocked Session.load_metadata_only to return an archived stub WITHOUT writing a
real sidecar file, and never set SESSION_DIR. The new stat-gate in
_state_projection_sidecar_metadata short-circuits on SESSION_DIR/{sid}.json
before consulting metadata, so those mocks were never reached -> archived=False.
This is NOT a production regression (in prod, load_metadata_only reads exactly
SESSION_DIR/{sid}.json and returns None when absent, so archived metadata only
exists when the file does — in which case the stat() succeeds and behavior is
identical). The divergence existed only at the tests' mock boundary.
Fix: both tests now write a real archived sidecar JSON under a monkeypatched
SESSION_DIR, exercising the actual production read path (strictly better
coverage than mocking load_metadata_only). Also documented the
SESSION_DIR-as-sole-source assumption on the cache function per Opus's
forward-looking note.