Release v3.19.3

• fix: the membrane fit guard checked the wrong box, and died opaquely when it had none.
  make_membrane_system's pre-embed guard exists to catch an over-condensed quilt before the
  protein is embedded into a box too small for it -- "turning a silent squeeze into a loud,
  diagnosable failure", per its own comment. It did neither, from v3.15.0 (ca6c0b11) onward.

  It read get_current_artifact('state'), but at that point in do() that key still holds the
  protein's state; the membrane's is quilt_state (every path registers it there -- symmetric
  grid, asymmetric grid, and prebuilt alike). Comparing the protein's box against the protein's
  footprint is 2 * xydist by construction and passes however far the membrane has condensed.
  Mostly it did not get even that far: the orient step registers only psf and pdb, so there was no
  xsc and the guard returned silently, having checked nothing.

  When a config did leave an xsc on the protein state -- a continuation, or a vacuum minimize --
  the second defect fired. cell_from_xsc is documented to return (None, None) for an xsc with
  no periodic cell, and a run with no periodic boundaries writes exactly that: an origin-only
  step o_x o_y o_z, 4 columns. The return was indexed unchecked, so the guard raised
  TypeError: 'NoneType' object is not subscriptable tens of lines from its cause -- after a
  3h52m quilt equilibration in the reported case.

  The guard now reads quilt_state, logs the two boxes it compared (it was previously silent
  unless it tripped, so there was no way to tell which box it had read), warns rather than passing
  mutely when no membrane xsc is available, and routes every cell_from_xsc call in the task
  through a helper that names the file and explains the 4-column case. cell_from_xsc also now
  returns (None, None) for a truncated or header-only file instead of raising EmptyDataError
  through its caller. The other call sites -- terminate.py, solvate.py, run_record.py,
  ringcheck.py, ring_resolve.py -- were already guarded and are unchanged.

  Guarded by tests/unit/test_tasks/test_make_membrane_decisions.py::TestMembraneSpansProteinGuard,
  which pins the artifact the guard reads (a membrane condensed below the protein must raise even
  when the protein's own state carries a comfortable box) and the cell-less failure message. All
  six fail on the unfixed code.

  Reported by the pestifer-sweep session, found while debugging an unrelated user config.