srxsync: lock candidate config at connect() in rustez, matching PyEZ

[fastrevmd-lab]

Summary

PyEZTransport takes the exclusive candidate lock in connect() and holds it until close(). RustezTransport deferred locking to the first load() call, which left a window on the push path (connect() → load()) where another session could mutate the candidate config while ours sat unlocked, and meant a read-only check (connect() + fetch(), no load()) never locked at all on rustez while it always locked on PyEZ (#7 documented this without fixing it).

This moves the rustez lock acquisition into connect() so both backends take and hold the lock at the same point in the session lifecycle. load() no longer lazily creates/locks the Config object — it uses the one set up in connect().

Changes

• srxsync/transport/rustez.py: connect() now creates the Config and calls lock() right after Device.open(), setting self._locked = True. load() no longer lazily locks.
• README.md: updated the "Locking differs between backends" note to describe the now-unified behavior.
• Tests (one per backend, per the issue):
  • tests/unit/test_transport_pyez.py::test_connect_locks_before_any_read — asserts PyEZ's existing lock-in-connect() ordering (open, then lock) as a baseline/regression guard.
  • tests/unit/test_transport_rustez.py::test_connect_locks_immediately_like_pyez — asserts rustez now locks in connect() (fails against the old code, where connect() never touched Config).
  • tests/unit/test_transport_rustez.py::test_load_does_not_relock — asserts load() reuses the existing lock rather than calling lock() again.

Verification

.venv/bin/ruff check .                                    # All checks passed!
.venv/bin/mypy srxsync                                     # Success: no issues found in 19 source files
.venv/bin/python -m pytest tests/unit -q                   # 73 passed

No new dependencies. No live device access used — verified against fixtures/mocks per existing test patterns in tests/unit/test_transport_{pyez,rustez}.py.

Closes #8

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