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/SimRNG.h

@@ -3,16 +3,19 @@
 #include <Utils.h>
 #include <cstdlib>
 #include <ctime>
+#include "SimInstance.h"
 
 // mesh::RNG implementation for the native sim build. Not cryptographically
-// strong (rand() under the hood) -- fine for a terminal demo; a real
-// two-device-messaging phase (Phase 3 of the sim plan) may want to swap
-// this for something seeded from the OS CSPRNG.
+// strong (rand() under the hood) -- fine for a terminal demo. Phase 3 mixes
+// in sim_instance_salt() so that two module instances of the SAME compiled
+// binary (the browser's two companion_radio instances) can't end up with
+// the same seed -- see SimInstance.h for why that's a real risk here, not
+// a hypothetical one.
 class SimRNG : public mesh::RNG {
 public:
   SimRNG() { }
   void begin() {
-    unsigned seed = (unsigned)time(NULL) ^ (unsigned)(uintptr_t)this;
+    unsigned seed = (unsigned)time(NULL) ^ (unsigned)(uintptr_t)this ^ sim_instance_salt();
     srand(seed);
   }
   void random(uint8_t* dest, size_t sz) override {