# Issue #48 fix — bilinear fallback triangle-pair test was swapped

## Diagnosis (from the diagnostic dump)

ACDREAM_DUMP_SCENERY_Z=1 in landblock 0xA9B3 around the user's spawn
showed:

- Every scenery line had `source=bilinear` (physics engine had not
  registered the landblock at hydration time — typical streaming race).
- The trees flagged as "floating" had multi-part setups whose lowest
  vertex extended into the ground per our calc (`partWorldZMin =
  groundZ - 3.825`) — i.e., per the diagnostic the tree should sit
  flush. The retail client at the same world coords does sit flush.
- Player Z at the same world coords (~91 m) came from the physics
  sampler (`TerrainSurface.SampleZ`), which is correct.

That meant the bilinear fallback in `GameWindow.SampleTerrainZ` was
producing a different ground Z than the visible terrain mesh and the
player physics path on sloped cells, so trees were placed at a
ground-Z that didn't match the terrain the player was walking on.

## Root cause

Two terrain Z samplers exist:

1. `AcDream.Core.Physics.TerrainSurface.SampleZ` (instance) — used by the
   physics engine for player + entity ground-snap. Triangle-aware,
   matches the visible terrain mesh.
2. `AcDream.App.Rendering.GameWindow.SampleTerrainZ` (private bilinear
   fallback) — used by scenery hydration when physics has not yet built
   a `TerrainSurface` for a streaming-in landblock.

Both choose a per-cell **diagonal** with the AC2D `FSplitNESW` formula
(constants `0x0CCAC033`, `0x421BE3BD`, `0x6C1AC587`, `0x519B8F25`).
Both then choose one of the cell's two **triangles** based on the
fractional position within the cell. **The fallback's two arms were
swapped** relative to the chosen diagonal:

| Diagonal | Correct dividing test | Correct triangles | Bilinear-fallback test (BUGGY) |
|---|---|---|---|
| `SWtoNE` (BL→TR, line y=x) | `tx > ty` | {BL,BR,TR} below / {BL,TR,TL} above | `s + t <= 1` (wrong — that's the SEtoNW test) |
| `SEtoNW` (BR→TL, line x+y=1) | `tx + ty <= 1` | {BL,BR,TL} below / {BR,TR,TL} above | `s >= t` (wrong — that's the SWtoNE test) |

On sloped cells the wrong triangle's plane gives a Z that disagrees
with the rendered terrain by up to ~1.5 m. Flat cells happen to mask
the bug because all four corners share one Z.

## Fix

1. Extract the correct triangle-picker math from `TerrainSurface.SampleZ`
   (instance) into a new public static method
   `TerrainSurface.SampleZFromHeightmap(byte[] heights, float[] heightTable,
   uint landblockX, uint landblockY, float localX, float localY)`.
   Same algorithm, but reads the four corner heights directly from the
   landblock's raw heightmap byte array instead of the pre-resolved
   instance cache. One source of truth for the triangle math.
2. Replace `GameWindow.SampleTerrainZ` body with a call to that static.
3. Conformance test in `tests/AcDream.Core.Tests/Physics/TerrainSurfaceTests.cs`
   exercising a sloped heightmap on both diagonals, asserting that the
   new static and the existing instance method return the same Z at the
   same `(localX, localY)` (especially at points near the cell diagonal
   where the previous bug manifested).

## Why this is the right fix

- Retail (`docs/research/named-retail/acclient_2013_pseudo_c.txt`) places
  scenery via `CLandBlock::get_land_scenes` (0x00530460) →
  `Plane::set_height(plane, &pos)` (0x0052f050), which projects the
  position onto the terrain triangle's plane. The split direction comes
  from the same `FSplitNESW` formula. Our `TerrainSurface.SampleZ` is a
  faithful port of that algorithm and is already used by physics; the
  bilinear fallback should be identical.
- The bug is purely in the fallback path. Player Z is unaffected.
- No retail behavior changes; only acdream consistency.
- "Don't break" constraints from the handoff are satisfied:
  - Player Z snap untouched (different code path).
  - Species that already render flush still do (their Z was correct
    on cells where bilinear and physics agreed; now it's correct
    everywhere).

## Pre-existing bugs out of scope

The user reports separate (X, Y) misplacement at other locations.
That's a different bug — likely in `SceneryGenerator`'s placement
math or one of the terrain-mesh / region tables — and outside the
scope of this fix. File as a follow-up issue.
