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.c

@@ -1,4 +1,5 @@
 #include "engine/input.h"
+#include "engine/debuglog.h"
 #include <string.h>
 
 static bool s_current[ACTION_COUNT];
@@ -196,6 +197,19 @@ bool input_key_held(SDL_Scancode key) {
     return s_key_state && s_key_state[key];
 }
 
+/* ── Debug log snapshot ───────────────────────────── */
+
+void input_get_snapshot(InputSnapshot *out) {
+    out->held = 0;
+    out->pressed = 0;
+    out->released = 0;
+    for (int i = 0; i < ACTION_COUNT && i < 8; i++) {
+        if (s_current[i])          out->held     |= (uint8_t)(1 << i);
+        if (s_latched_pressed[i])  out->pressed  |= (uint8_t)(1 << i);
+        if (s_latched_released[i]) out->released |= (uint8_t)(1 << i);
+    }
+}
+
 void input_shutdown(void) {
     /* Nothing to clean up */
 }