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>

examples/companion_radio/MyMesh.h

@@ -108,6 +108,17 @@ public:
   void loop();
   void handleCmdFrame(size_t len);
   bool advert();
+#ifdef SIM_PLATFORM
+  // Phase 3 sim-only test hook: same as advert() but FLOOD-routed (like the
+  // real phone-app CMD_SEND_SELF_ADVERT command's flood=1 branch) instead of
+  // zero-hop, so a JS test harness can make an instance's identity actually
+  // propagate through an intermediate repeater instance -- advert() alone
+  // (zero-hop) never leaves the immediate ether link. Not reachable from any
+  // real hardware build (no phone app exists in the sim to send the real
+  // CMD_SEND_SELF_ADVERT command over), so this is additive/dead code
+  // everywhere else, not a behavior change.
+  bool advertFlood();
+#endif
   void sendNodeDiscoverReq();
   void enterCLIRescue();