ArtillerySimulator polish — finished-product pass

name: ArtillerySimulator polish overview: Fix ballistic terrain impact and visible arc physics, then raise frame rate and UX (aim preview, gun framing, toggle camera) while keeping the educational wireframe aesthetic. todos:

• id: fix-projectile-sweep

content: "Refactor ProjectileRun: integrate-then-sweep, sub-step cap (~2m), correct impact time, denser trail" status: completed

• id: terrain-draw-perf

content: "TerrainWorld: 64 collision mesh, fast grid-line or 32x32 draw mesh with height tint" status: completed

• id: artillery-camera

content: ArtilleryCamera with fixed + chase modes, C toggle, smooth blend status: completed

• id: gun-and-preview

content: Scale up GunModel draw; add BallisticArcPreview in Ready state status: completed

• id: hud-validate

content: HUD in-flight stats + cam hint; manual validate impact, FPS, flat vacuum sanity status: completed isProject: false

What is already right

• App structure (`Program.cs`, `ArtillerySimulatorGame`) matches other dogfood apps.
• Libraries are wired correctly: ProjectileBallisticSimulation, BvhStaticWorld, BallisticsQueries.SweepProjectileSphere.
• Gun preset, HUD, fire/reset flow, and impact marker drawing exist.
• Screenshot symptoms (straight trail through hills, Impact —, ~27 FPS, weak gun presence) match fixable bugs and rendering cost, not a wrong product direction.

Root causes (from code + physics docs)

flowchart TD
  subgraph bug [Current ProjectileRun.Advance]
    A[state at t] --> B["sweep: displacement = v_old * dt"]
    B -->|miss| C["Step: p = p + v_new * dt"]
    B -->|hit| D[RecordImpact]
  end
  subgraph fix [Correct segment]
    E[state at t] --> F["Step to candidate next"]
    F --> G["sweep: displacement = p_next - p"]
    G -->|hit| H[Impact at fraction along segment]
    G -->|miss| I[Accept next state]
  end

Your choices for this pass: fast wireframe terrain (not solid mesh) and C toggles fixed vs chase camera.

Phase 1 — Correct ballistic + terrain collision (must-have)

Refactor [`ProjectileRun.Advance`](d:\novolis\novolis-dogfooding\apps\ArtillerySimulator\Game\ProjectileRun.cs) to match the integrator and `CollisionSweepScenarioTests` pattern:

1. candidate = sim.Step(_state, dt, env)
2. displacement = candidate.Position - _state.Position
3. SweepProjectileSphere from _state.Position with that displacement
4. On hit:

• frac = hit.Distance / |displacement|
• Impact position = _state.Position + displacement * frac
• Impact time = _state.TimeSeconds + dt * frac (fix currently using pre-impact time in RecordImpact)
• Impact velocity ≈ _state.Velocity (acceptable for HUD; optional: one partial Step only if needed later)

1. On miss: _state = candidate

Sub-step sweeps (documented mitigation, stay in app — no library change required):

• Cap max sweep length (e.g. 2 m or min(2, |displacement|)).
• If |displacement| > cap, split into ceil(|disp|/cap) substeps: integrate one sub-dt, sweep, stop on hit.
• Keep physics dt = 1/120; run multiple substeps per frame in `ArtillerySimulatorGame.Update` until a budget (e.g. 16–32 substeps/frame) instead of fixed 4 blind loops.

Trail sampling: append trail point every substep (or every 2nd) while in flight so the arc reads clearly at 60 FPS.

Acceptance: firing charge 2 at ~45° over hills shows a curved trail, stops on surface, HUD shows range/TOF/speed; flat + vacuum still logs ~2*vx*vy/g sanity line.

Phase 2 — Terrain draw performance (wireframe, fast)

In `TerrainWorld`:

• Collision mesh: keep 64×64 (unchanged BVH).
• Draw mesh: separate 32×32 heightfield (or draw only grid lines: (GridDraw+1) polylines in X and Z ≈ 130×2 segments — far fewer than 24k triangle edges).
• Keep height-based color (_wireLow / _wireHigh).
• Optional: skip drawing every other line when FPS < 45 (only if needed after grid-line draw).

Acceptance: FPS ≥ 55 on typical hardware with terrain on.

Phase 3 — Camera + gun readability

Replace static [`OverviewCamera`](d:\novolis\novolis-dogfooding\apps\ArtillerySimulator\Game\OverviewCamera.cs) with `ArtilleryCamera`:

• Smooth lerp on target/eye (0.1–0.15s time constant) to avoid jitter.
• Persist mode in game state; show Cam fixed / Cam chase on HUD.

Gun draw in `GunModel.Draw`:

• Add a ground plate (2–3 m triangle or cross) at pivot so the piece reads at map scale.
• Thicker tube line + slightly larger muzzle wire.
• Draw pivot glow only in Ready (reduce clutter in flight).

Phase 4 — UX: aim preview + HUD polish

Aim preview (Ready only) — new small helper e.g. BallisticArcPreview.cs:

• Clone current ProjectileBallisticSimulation + environment from `GunModel`.
• Integrate up to 8–12 s or until below local terrain sample (TerrainWorld.SampleHeight) — no BVH for preview (cheap).
• Draw dimmer color (Color ~50% alpha of trail) as polyline from muzzle.
• Update when elevation/azimuth/charge/drag changes.

HUD (`SimulationHud.cs`):

• In flight: show Alt, Spd, Time live.
• Show camera mode; controls line: add `C` cam.
• Keep non-violent disclaimer.

Shot flow:

• R / F already reset — ensure they reset camera blend target to gun.
• After impact, stay in Impacted until Space (already); optionally auto-clear trail on next aim nudge (low priority).

Phase 5 — Validation (dogfood, no new package yet)

Defer to physics library (only if a second app needs it): SimulateProjectileUntilTerrainHit wrapping sub-step sweep loop — not required for this polish pass.

Files to touch (focused)

No edits to `artillerysimulator_dogfood_b3198318.plan.md`.

Out of scope (later)

• Solid shaded terrain / DrawMesh material pipeline in Raylib facade.
• Real charge tables, targets, explosions, ImGui panels.
• Library extraction of terrain-impact loop until another consumer appears.