feat(strate): passages aim at open space -- for half the archetypes, measurably

Tier 1. GeneratePassages placed every passage at a random angle, distance and
reach, never consulting the destination strate. The carved tube is air by
construction, so nothing looked wrong -- but whether the lower mouth joined the
destination's connected cave space was luck. A player could descend and arrive
in a sealed pocket. The primordial law was a hope.

VF_SuggestOpenPointZ is a pure free function of (archetype, params, seed, XY):
no operator stack, no StrateManager, no cache. That is deliberate. This runs
inside Initialize, where the layout is half-built and LayoutVersion is in
flight, so anything holding a manager pointer there is a re-entrancy trap.

Coverage is four archetypes of eight, and the number is the deliverable:
7 passages, 3 aimed at generator-confirmed open space with 48/48 ring samples
air, 4 falling back. The hole is not closed. It is measured, where this morning
it was invisible.

Two corrections from Codex, both mine and both would have shipped confident
nonsense: room placement is seeded by MakeStrateSeed, not the passage salt, so
my version would have queried a cave graph that does not exist; and slabs need
the world seed because that is what GetSlabDensity uses.

And one of mine caught in review: the first test sampled GetDensityAt at the
mouth and asserted air, which is true by construction because the passage carve
is a structural-post invariant. It could not fail. Replaced with a 16-point
ring outside the carve radius requiring 8/16 air -- a proxy for connectivity,
not a proof. The proof is a flood fill, which is Tier 2.

Logging: my first spec asked for a per-passage warning on fallback. It fired
572 times and turned 14 clean tests yellow, which is precisely the anti-pattern
OPSTACK-HANDOFF.md warns about -- a warning that always means "this is fine"
teaches people to ignore warnings. Now one sorted Log line per layout build.

Built clean, suite green: 17 succeeded, 0 with warnings, 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DPUuCtn5zTLui6WybLbkNb

ARCHITECTURE.md

@@ -290,6 +290,16 @@ is a simple straight tube. Placement, reach, and shape draws for inter-strate pa
 connection count cannot shift any later passage. Global passage settings were removed from
 `VoxelSettings`.
 
+After a passage's XY is selected, `GeneratePassages` asks the **destination** strate for a lower-mouth
+height through the pure free function `VF_SuggestOpenPointZ` (`VoxelCaveMorphology`). TunnelNetwork and
+Underwater use the nearest hash-placed room centre, with `MakeStrateSeed(world-seed, strate-index)`
+matching the room graph's existing identity; FlatPlain and CrystalChamber recompute their slab void
+band and reject column-overlap points. Maze, SurfaceWorld, VerticalShafts, and FloatingIslands remain
+explicitly unanswerable for this tier: they keep the historical random reach and emit a warning. This
+query is source-level only — it does not construct an operator stack, touch a cache, or call back into
+`UVoxelStrateManager` while `Initialize` is building the layout. The resulting lower endpoint is
+clamped to the destination's seal-free interior.
+
 ### 8.9 Carving — brush shapes + editor controls
 `FVoxelModification` has `EVoxelBrushShape {Sphere,Box,Capsule}` + `BoxExtent`/`CapsuleEnd`/
 `Falloff` + `GetWorldBounds`. `UVoxelDiffLayer::GetDensityOffset` switches per shape; chunk