feat(sim): two-device messaging + repeater relay over a JS ether

Ports examples/simple_repeater to variants/sim/ (new sim_simple_repeater
native env + build_wasm_repeater.sh) and adds a JS "ether"
(variants/sim/web/mesh.html) that bridges two real companion_radio WASM
instances through a real simple_repeater instance in a strict A<->R<->B
topology (no direct A-B link), proving genuine relay routing rather than
a shortcut.

Also fixes multi-instance issues Phase 2's single-instance design never
surfaced: SimDisplayDriver's canvas context/id caching was keyed on a
single global instead of per-instance, and both wasm builds were missing
_malloc/_free/HEAPU8 runtime exports needed for the ether to poke bytes
into an instance's memory.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

variants/sim/target.h

@@ -11,6 +11,12 @@
 #include "SimMainBoard.h"
 #include "SimRTCClock.h"
 #include <helpers/SensorManager.h>
+// Included here (rather than only where it's used) so its JS-facing
+// sim_location_set() EMSCRIPTEN_KEEPALIVE export actually gets compiled
+// into every SIM_PLATFORM target that includes target.h (companion_radio
+// AND simple_repeater) -- an inline function nobody #includes never gets
+// emitted at all, KEEPALIVE or not.
+#include "SimLocationProvider.h"
 
 #ifdef DISPLAY_CLASS
   #include "SimDisplayDriver.h"