Add analytics integration with Horchposten backend

Implements session-based analytics tracking that sends gameplay
stats to the Horchposten API. Adds stats.{c,h} for accumulating
per-session metrics (kills, deaths, shots, dashes, jumps, pickups,
damage, time) and analytics.{c,h} with EM_JS bridge for fetch()
calls to the backend. Client ID is persisted in localStorage.
Session start/end hooks are wired into all game lifecycle events
(level transitions, restart, quit, tab close via sendBeacon).
Analytics URL/key are configured via data attributes on the canvas
container. Non-WASM builds compile with no-op stubs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/game/analytics.h

@@ -0,0 +1,17 @@
+#ifndef JNR_ANALYTICS_H
+#define JNR_ANALYTICS_H
+
+#include "game/stats.h"
+
+/* Initialize analytics subsystem (load/generate client_id).
+ * No-op on non-WASM builds. */
+void analytics_init(void);
+
+/* Start a new analytics session. Sends POST to the backend. */
+void analytics_session_start(void);
+
+/* End the current analytics session with final stats.
+ * end_reason: "death", "quit", "timeout", or "completed". */
+void analytics_session_end(const GameStats *stats, const char *end_reason);
+
+#endif /* JNR_ANALYTICS_H */