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/platformio.ini

@@ -62,3 +62,79 @@ build_src_filter =
   +<../variants/sim/thirdparty/cayennelpp/*.cpp>
   +<../examples/companion_radio/*.cpp>
   +<../examples/companion_radio/ui-new/*.cpp>
+
+; Phase 3: native build of the real examples/simple_repeater app logic
+; (MyMesh/no UI) against the same variants/sim/ interfaces as
+; [env:sim_companion_radio] above. No DISPLAY_CLASS defined -- a repeater
+; doesn't need one (examples/simple_repeater/main.cpp already gates its
+; UITask/display use behind #ifdef DISPLAY_CLASS, same as every real
+; headless-repeater hardware env does), which also means UITask.cpp/.h
+; never need to be ported/compiled for this target at all.
+;
+; Everything else mirrors [env:sim_companion_radio] deliberately closely
+; (same src/*.cpp list, same real hardware envs' own convention of sharing
+; one broad src/helpers/*.cpp filter across companion_radio/simple_repeater/
+; room_server -- see e.g. variants/ebyte_eora_s3/platformio.ini's
+; [env:Ebyte_EoRa-S3] base + its "_Repeater" env only adding
+; "+<../examples/simple_repeater>" on top) -- a few of those .cpp files
+; (BaseChatMesh.cpp, ConfigSerializer.cpp, DeviceDiag.cpp) aren't actually
+; reachable from simple_repeater's own code, but compiling them in unused is
+; harmless and keeps this env a straightforward diff against the one above
+; rather than a hand-pruned, easy-to-drift-out-of-sync list.
+[env:sim_simple_repeater]
+platform = native
+build_type = debug
+build_flags =
+  -std=c++17
+  -g
+  -funsigned-char
+  -D SIM_PLATFORM
+  -D MESH_DEBUG=0
+  ; Real hardware repeater envs get this from [arduino_base] (which this
+  ; from-scratch platform=native env deliberately doesn't extend -- see the
+  ; header comment on [env:sim_companion_radio] above); set it explicitly so
+  ; the repeater actually broadcasts its own identity on boot, same as a
+  ; real one would.
+  -D ENABLE_ADVERT_ON_BOOT=1
+  -I variants/sim/arduino
+  -I variants/sim
+  -I variants/sim/thirdparty/crypto
+  -I variants/sim/thirdparty/cayennelpp
+  -I variants/sim/thirdparty/arduinojson
+  -I lib/ed25519
+  -I src
+  -I examples/simple_repeater
+build_src_filter =
+  -<*>
+  +<../src/Dispatcher.cpp>
+  +<../src/Identity.cpp>
+  +<../src/Mesh.cpp>
+  +<../src/Packet.cpp>
+  +<../src/Utils.cpp>
+  +<../src/helpers/AdvertDataHelpers.cpp>
+  +<../src/helpers/BaseChatMesh.cpp>
+  +<../src/helpers/ClientACL.cpp>
+  +<../src/helpers/CommonCLI.cpp>
+  +<../src/helpers/ConfigSerializer.cpp>
+  +<../src/helpers/DeviceDiag.cpp>
+  +<../src/helpers/IdentityStore.cpp>
+  +<../src/helpers/RegionMap.cpp>
+  +<../src/helpers/StaticPoolPacketManager.cpp>
+  +<../src/helpers/TransportKeyStore.cpp>
+  +<../src/helpers/TxtDataHelpers.cpp>
+  +<../lib/ed25519/*.c>
+  +<../variants/sim/*.cpp>
+  +<../variants/sim/thirdparty/crypto/*.cpp>
+  +<../variants/sim/thirdparty/cayennelpp/*.cpp>
+  +<../examples/simple_repeater/*.cpp>
+  ; UITask.cpp uses real Arduino GPIO constants (HIGH/LOW/digitalRead(),
+  ; gated on real #if PIN_USER_BTN/UI_HAS_JOYSTICK defines this env never
+  ; sets) directly at file scope in its button-polling code, unconditionally
+  ; -- unlike companion_radio's ui-new/UITask.cpp, it isn't written to
+  ; compile cleanly with no display at all. Since DISPLAY_CLASS is
+  ; deliberately undefined for this headless repeater env (main.cpp's own
+  ; `#ifdef DISPLAY_CLASS` already skips constructing a UITask instance
+  ; entirely), this translation unit is simply never needed -- exclude it
+  ; rather than porting GPIO stubs into variants/sim/arduino/Arduino.h for
+  ; code that would never run.
+  -<../examples/simple_repeater/UITask.cpp>