Fix unrealistic range-limit impacts

name: Fix range-limit impacts overview: Range-limit stops currently record impact at the in-air point where the trajectory crosses the map box (e.g. Y≈2428 m at X=2500), producing impossible "Beyond range" sky impacts. Fix by removing vertical BVH walls, projecting edge impacts onto terrain, and clarifying HUD. todos:

• id: project-ground

content: Add TerrainWorld.ProjectOntoTerrainSurface; use in all BeyondRange impact paths status: completed

• id: remove-walls

content: Remove vertical wall triangles from BVH; keep gold boundary draw only status: completed

• id: preview-hud

content: BallisticArcPreview ground endpoint at edge; HUD Range limit (map edge) wording status: completed

• id: validate-build

content: Release build; verify high-angle shot shows ground Y at map edge status: completed isProject: false

What you are seeing

Your HUD: 46.5° elev, 560 m/s, TOF 6.4 s, impact at (2500, 2428, 906), Beyond range.

That is physically consistent with the current code, but wrong for an artillery demo:

sequenceDiagram
  participant Gun
  participant Traj as Trajectory
  participant Box as XZ_box_exit
  Gun->>Traj: Fire +X at high quadrant
  Traj->>Box: Crosses X=2500 after ~6.4s still ascending
  Note over Traj: Y ≈ 2400 m (not ground)
  Box->>Traj: TrySegmentLeavesRange records air point
  Traj->>HUD: Beyond range at altitude 2428 m

At 46.5°, vertical velocity is still up after 6.4 s; horizontal travel reaches the 2.5 km east edge while the round is thousands of meters above the hills (~20–45 m). `TryRangeOrGroundHit` records boundaryHit as-is (line 153–156), so impact Y stays at flight altitude.

Vertical BVH walls in `TerrainWorld.AddRangeWalls` (450 m tall) can also register mesh hits in the air if the segment test does not run first — redundant and confusing.

Fix strategy

1. Ground-projected range limit (core)

Add on `TerrainWorld`:

public Vector3 ProjectOntoTerrainSurface(Vector3 p, float surfaceEpsilon = 0.05f)
{
  var x = Math.Clamp(p.X, 0f, ExtentMeters);
  var z = Math.Clamp(p.Z, 0f, ExtentMeters);
  var y = SampleHeight(x, z) + surfaceEpsilon;
  return new Vector3(x, y, z);
}

Use everywhere a range limit is recorded (`ProjectileRun`):

• TryRangeOrGroundHit — after TrySegmentLeavesRange, replace boundaryHit with terrain.ProjectOntoTerrainSurface(boundaryHit)
• TryTerrainHit — boundary branch (lines 217–222)
• TryFallbackContact — OOB endpoint: clamp + project before RecordImpact

Rename reason in HUD copy only (keep enum BeyondRange or add RangeLimit alias): show "Range limit (map edge)" instead of bare "Beyond range".

2. Remove vertical walls from collision mesh

In `TerrainWorld.BuildMeshes`:

• Delete AddRangeWalls call and method (or stop adding wall triangles to BVH).
• Keep the gold wire rectangle (DrawRangeBoundary) as the visual limit only.
• Terrain + heightfield + logical XZ segment exit remain the collision story.

This prevents TerrainMesh hits on invisible walls in the sky.

3. HUD clarity ([`SimulationHud.cs`](d:\novolis\novolis-dogfooding\apps\ArtillerySimulator\Game\SimulationHud.cs))

• Impact line: Range limit (map edge) when Reason == BeyondRange.
• Coordinates line: label ground impact — values should show Y ≈ terrain height (tens of m), not thousands.
• Optional second stat: Max ord {apex} m deferred (out of scope unless easy from trail max Y).

4. Aim preview ([`BallisticArcPreview.cs`](d:\novolis\novolis-dogfooding\apps\ArtillerySimulator\Game\BallisticArcPreview.cs))

When preview crosses the box, add a final point at ProjectOntoTerrainSurface(boundaryHit) so the ghost arc ends on the ground at the edge, not in the air.

5. Optional tuning (only if still frustrating after fix)

If many shots still end at the east edge before descending, bump `SimulationUnits.ExtentMeters` from 2500 → 4000 (keep 128×128 collision). Not required for the correctness fix; mention in validation.

Validation

Build Release, manual fire once.

Files to change

No physics library changes.