Add game state debug log with binary ring buffer

Implement src/engine/debuglog module that records a comprehensive
snapshot of game state every tick into a 4 MB in-memory ring buffer.

Activated by --debug-log command-line flag. Press F12 during gameplay
to dump the ring buffer to a human-readable debug_log.txt file. The
buffer also auto-flushes every 10 seconds as a safety net.

Each tick snapshot captures: input state (held/pressed/released bitmasks),
full player state (position, velocity, health, dash, aim, timers),
camera position, physics globals, level name, and a variable-length
list of all active entity positions/velocities/health.

New files:
- src/engine/debuglog.h — API and snapshot data structures
- src/engine/debuglog.c — ring buffer, record, and dump logic

Modified files:
- include/config.h — DEBUGLOG_BUFFER_SIZE constant
- src/engine/input.h/c — input_get_snapshot() to pack input bitmasks
- src/engine/core.c — debuglog_record_tick() call after update
- src/main.c — CLI flag, init/shutdown, F12 hotkey, set_level calls

Closes #19

src/engine/input.h

@@ -50,4 +50,10 @@ int input_mouse_scroll(void);
 bool input_key_pressed(SDL_Scancode key);
 bool input_key_held(SDL_Scancode key);
 
+/* Pack current input state into a compact bitmask snapshot.
+ * Used by the debug log to record per-tick input without
+ * exposing internal arrays. */
+typedef struct InputSnapshot InputSnapshot; /* defined in debuglog.h */
+void input_get_snapshot(InputSnapshot *out);
+
 #endif /* JNR_INPUT_H */