v2: kick off v2-dev — version constant + roadmap

Freeze the classic line on main (tagged v1.0-classic) and open the v2
development branch. Add a firmware version constant (2.0.0-dev) reported in
the boot banner so the daemon can tell classic from v2, and document the
phased v2 plan in docs/v2-roadmap.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/main.cpp

@@ -7,6 +7,7 @@
 #include "data.h"
 #include "ui.h"
 #include "ble.h"
+#include "version.h"
 #include "splash.h"
 #include "usage_rate.h"
 #include "idle.h"
@@ -180,7 +181,7 @@ extern "C" void board_init(void);
 void setup() {
     Serial.begin(115200);
     delay(300);
-    Serial.println("{\"ready\":true}");
+    Serial.printf("{\"ready\":true,\"fw\":\"%s\"}\n", CLAWDMETER_VERSION);
 
     board_init();