feat(opstack): VerticalShafts' connector branch never tested a connector -- test the real capsules

Jahni's call: bank T1.d at 11/40 and take VerticalShafts rather than squeeze the
Perlin sup. Right call -- a clearly-scoped defect with no hole-risk maths, on an
archetype that was getting nothing.

The defect is stated in its own comment. FShaftFieldSource::EffectOverBox ended
with a "connectors" branch that never looks at a connector: it returns CarveOnly
because a shaft EXISTS within Spacing*1.6 + Pad. At ShaftSpacing 55 and
ShaftDensity 0.6 that box spans ~4x4 cells and ~10 shafts, so the condition is
true essentially everywhere -- exactly the reported 0 proved of 60. Conservative,
never wrong, completely sterile; the same shape as the worm's unconditional
CarveOnly one archetype over.

It now rebuilds the connectors the way GetCells does and tests the real capsule.
Two things had to be right and both were read in the source rather than assumed:

- The enumeration is a superset. Eval reads connectors from the 3x3 of ITS
  query's cell, so any pair visible from a point in the box has both shafts in
  the 3x3 of some cell the box touches, i.e. in [box cells] +- 1 -- the range
  swept here. Pairs no query ever sees may be produced: extra CarveOnly, never a
  hole.
- The pair order matches, so the hash matches. VoxelHash::Pair is fed in
  insertion order and GetCells inserts over (dy, dx), row-major; this sweeps
  (cy, cx), and row-major order restricted to a sub-grid preserves the relative
  order of two cells. So the same pair gets the same hash WITHOUT assuming
  Pair() is symmetric -- which was never verified and now need not be.

The capsule test splits the axes because a connector is HORIZONTAL at height Zc:
Z is exact, XY is point-to-segment from the box centre minus the XY half
diagonal. Tighter than the 3-D half-diagonal the tunnel version had to settle for.

Also the same sampler trap, caught before the build this time: tile XY was drawn
from +-48 voxels against ShaftSpacing 55 -- under one period of the pattern,
identical in kind to the +-32-vs-80 bug that cost the tunnel test three runs.
Widened to +-440, and the report prints its own extent in units of ShaftSpacing.

The safety net was already there and is untouched: this test brute-forces the
full lattice of every proved tile, both hypotheses, and AddErrors on the first
violation. If the superset or hash-order argument is wrong, the assertion fails
rather than a player falling through the floor.

Unbuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

OPSTACK-PROGRESS.md

@@ -3196,3 +3196,73 @@ appetite, not a technical unknown, so it goes to Jahni rather than getting done
 **11 of 40 tiles (27.5 %) proved AllSolid at production defaults, 14641 voxels brute-forced,
 0 violations.** The dense fixture correctly proves nothing. Every verdict is checked voxel by voxel,
 so the risk of the whole feature is bounded by a test that runs on every build.
+
+## 2026-07-29 — T1.d banked at 11/40. VerticalShafts: the connector branch never tested a connector.
+
+Jahni's call: bank the tunnel result and take VerticalShafts rather than squeeze the Perlin sup.
+Right call — the shaft fix is a clearly-scoped defect with no hole-risk maths, on an archetype that
+was getting nothing.
+
+### The defect, and it is stated in its own comment
+
+`FShaftFieldSource::EffectOverBox` ends with a "connectors" branch that never looks at a connector:
+
+```cpp
+const FBox ConnBox = VoxelBox.ExpandBy(Spacing * 1.6f + Pad);
+for (cells in ConnBox)
+    if (RollShaft(cx, cy, Sh)) { return EVoxelOpEffect::CarveOnly; }   // prudent
+```
+
+It returns `CarveOnly` because a shaft **exists** somewhere in the neighbourhood. Against the
+defaults — `ShaftSpacing = 55`, `ShaftDensity = 0.6`, `Pad ≈ 11 + ExtraReach` — that expanded box
+spans roughly 4×4 cells and therefore ~10 shafts. **The condition is true essentially everywhere**,
+which is exactly the reported `0 proved of 60`. Conservative, never wrong, and completely sterile —
+the same shape as the worm's unconditional `CarveOnly`, one archetype over.
+
+### The fix: rebuild the connectors the way `GetCells` does, and test the real capsule
+
+Two things had to be right, and both were checked in the source rather than assumed:
+
+- **The enumeration is a superset.** `Eval` reads connectors from the 3×3 neighbourhood of *its
+  query's* cell. So any pair visible from a point in the box has both shafts inside the 3×3 of some
+  cell the box touches ⇒ both lie in `[box cells] ± 1`, which is precisely the range swept. Pairs
+  that no query ever sees may be produced — extra `CarveOnly`, never a hole.
+- **The pair order matches, so the hash matches.** `VoxelHash::Pair(A…, B…)` is fed in insertion
+  order, and `GetCells` inserts over `(dy, dx)` — row-major. The sweep here is `(cy, cx)`, the same
+  order, and row-major order restricted to a sub-grid preserves the relative order of any two cells.
+  So the same pair gets the same hash **without assuming `Pair()` is symmetric** — which was never
+  verified and now does not need to be.
+
+The capsule test splits the axes because a connector is a **horizontal** capsule at height `Zc`:
+Z is exact (`RMinZ > Zc + Reach || RMaxZ < Zc - Reach` ⇒ skip), XY uses point-to-segment from the box
+centre minus the XY half-diagonal. Far tighter than a 3-D half-diagonal, which is what the tunnel
+version had to settle for.
+
+### And the same sampler trap, caught this time before the build
+
+The shaft test drew tile XY from `RandRange(-6,6)*8` = **±48 voxels, against `ShaftSpacing = 55`** —
+less than one period of the pattern. Identical in kind to the ±32-vs-80 bug that cost the tunnel test
+three runs. Widened to ±440 (8 periods), and the report now prints its own extent in units of
+`ShaftSpacing`, so it cannot go unnoticed again.
+
+**The safety net was already there and is untouched:** this test brute-forces the full lattice of
+every proved tile against `EvalMC`, both hypotheses, and `AddError`s on the first violation. So the
+fix is checked by construction — if the superset argument or the hash-order argument is wrong, the
+existing assertion fails rather than a player falling through the floor.
+
+### Ready to build. Compile-error spots
+
+1. `VF_DistPointSegment2D` — new helper next to `VF_NearCaveSurface` in the anonymous namespace.
+2. Both new helpers now use `KINDA_SMALL_NUMBER`, matching the ~6 existing uses in the plugin; they
+   were briefly written with the `UE_`-prefixed spelling, which nothing else here uses.
+3. The shaft test hoists `SpanCells` / `SpanVoxels` **outside** the tile loop — the report needs
+   them, and `Extent` is loop-local. (That exact scoping slip happened in the tunnel test two rounds
+   ago; caught here before the build rather than after.)
+4. Format string is 6 specifiers / 6 arguments — counted.
+
+### What to read
+
+`Box verdicts over 60 VerticalShafts tiles` — **0 is the number to beat**, and `%d violations` must
+stay 0. If it is still 0 proved, the warning now says what to check *first*: `ExtraReach` inflates
+both remaining tests, so its value against `ShaftMaxRadius` is the thing to look at before touching
+either test — not a re-derivation from scratch.