Implements spec revision 4 (green r4). §3.1 — a legacy open_spans/open_to cut
never zeroes the atom that carries an existing contour opening: the opening's
edges become atom boundaries, the door keeps its real wall and the zero run
continues on both sides. §3.2 — an ambiguous carrier resolves
deterministically: current host, then distance, thicker cm, smaller id.
§3.3 — an opening with no usable carrier persists unhosted: a valid degraded
v9 state, inert in the physics, rendered by its own x/y, kept by later writes
and re-placeable in the editor (backend schema accepts it). §3.4 — the
initial migration never throws over an opening; a post-v9 write that LOST its
carrier keeps the fail-closed opening-host refusal. The Python migration
mirror implements the same rules with byte-identical output (verified on the
span+door fixture including the segment id).
The new smoke replays #316 end to end: a conflicted space no longer blocks
drawing on an empty plan. The golden scene span-over-door-migrated-dark
renders the migrated fixture pinned byte-for-byte to the real writer; two
gate mutants revert §3.1 and §3.4 and are red by execution.
Issue: #316
User-Visible: yes
Codex