Add new level transition state machine

TODO.md

@@ -160,11 +160,25 @@ The spacecraft fly-in animation should only play on surface levels (moon01,
 mars01, etc.). Interior/base levels (mars02, mars03, generated mars_base,
 generated station) should skip it — the player is already indoors.
 
-## New level transition styles: elevator and teleporter
-Two new transition animations to complement the spacecraft fly-in:
-- **Elevator** — Doors slide shut, brief pause (screen shake / rumble),
-  doors slide open onto the new level. Good for base/station interior
-  transitions (mars02 → mars_base, between generated station levels).
-- **Teleporter** — Energy charge-up effect around the player, flash/warp
-  distortion, player materialises in the new level. Good for cross-planet
-  jumps or generated-to-handcrafted transitions.
+## ~~New level transition styles: elevator and teleporter~~ ✓
+Implemented: `src/game/transition.h` / `transition.c` module with two-phase
+transition state machine (outro on old level → level swap → intro on new level).
+
+- **Elevator** — Two horizontal doors slide inward from top/bottom (0.6 s),
+  hold closed with screen-shake rumble (0.3 s), then slide apart on the new
+  level (0.6 s). Smooth ease-in-out motion, dark gray industrial color,
+  bright seam at the meeting edge. Used for base/station interior transitions.
+- **Teleporter** — Scanline dissolve: 3 px-tall horizontal bands sweep
+  across the screen in alternating directions with staggered top-to-bottom
+  timing (0.5 s), then white flash (0.15 s). Intro reverses the sweep
+  bottom-to-top (0.5 s). Uses `teleport.wav` sound effect.
+
+New `MODE_TRANSITION` game state in `main.c` pauses gameplay during the
+animation. Level-load dispatch extracted into `dispatch_level_load()` helper,
+called both from instant transitions and from the transition state machine.
+
+New `.lvl` directives `TRANSITION_IN` and `TRANSITION_OUT` with values
+`none`, `spacecraft`, `elevator`, `teleporter`. Parsed in `tilemap.c`,
+saved by editor and level generator dump. All three procedural generators
+(generic, station, mars_base) set `TRANS_ELEVATOR` for interior themes.
+Handcrafted levels updated: mars02, mars03, level01, level02.