test: convert identity-hydrate tests to in-process
Replaces six per-test python subprocess invocations
(`subprocess.run([sys.executable, "-c", "from think.tools.sol import
app; app()"])`) with direct calls to `_hydrate()`. monkeypatch sets
SOLSTONE_JOURNAL via the journal_path fixture; SOL_SKIP_SUPERVISOR_
CHECK is no longer needed because the unit-level helper bypasses
require_solstone() entirely.
Same six assertions, same behavior coverage. 6 tests run in 0.07s
instead of multiple seconds of cold-import-per-subprocess. No more
risk of the 15s global timeout biting a busy box.