feat: camera zoom, day/night cycle, tooltip, crosshair

- Camera zoom: +/- keys adjust cell size 4-24px, grid_w/h recalculated
  Works in both graphics and ascii renderers via GpuRenderer trait
- Day/night cycle: ambient_light_at_tick() modulates brightness by sine
  Applied to terminal renderer (CPU lighting path)
- Tooltip: shows material name + temperature at mouse cursor position
  Tracked via CursorMoved event, world coords computed from screen pos
- Crosshair: dotted line from player to mouse cursor, fading alpha
  Shows aiming trajectory for projectile combat

All 185 tests + 14 scenarios pass. 162 FPS benchmark.

AGENTS.md

@@ -88,6 +88,7 @@ GPU (`--mode ascii` / `--mode graphics`):
 - `r` — drop first inventory item
 - `1`–`0` / `x` — paint material brush
 - `y` / `u` / `i` / `o` — move camera offset
+- `+` / `-` — zoom in / out
 - `m` — toggle audio
 - `q` / `esc` — quit