feat(sim): sim_radio_get_params export + fix identity-collision RNG bug

meshcore-solo-site's new cross-visitor relay bridge (Phase 3) needs a way
to read a sim instance's live freq/bw/sf/cr so a WebSocket relay server
can fan traffic out only to other visitors tuned to the same params,
mirroring real LoRa channel isolation. Added sim_radio_get_params()
(examples/companion_radio/main.cpp), reading NodePrefs directly -- the
real live source of truth, since SimRadio::setParams() has always been a
complete no-op (accepts and discards its arguments). Out-params via
pointer, same buffer-pointer idiom sim_radio_poll_tx() already uses;
build_wasm.sh needed HEAPF32/HEAP32 added to EXPORTED_RUNTIME_METHODS so
JS can read them back.

While testing the relay bridge with two independent browser contexts, hit
a real bug: sim_instance_salt() (SimInstance.h) is a pure function of the
simInstanceTag STRING ('hero'/'B'/'R'), so it's only useful for telling
apart same-tab instances -- it's identical for two genuinely different
visitors who both boot a 'hero' instance. Combined with SimRNG::begin()'s
other seed ingredients -- time(NULL) (1-second resolution) and a WASM
heap pointer (no ASLR inside the sandbox, so it's fully deterministic
across independent boots of the same binary) -- two different visitors
landing on the same wall-clock second reliably generated byte-identical
Ed25519 identities (confirmed: two fresh browser contexts launched
together produced provably identical advert packets end to end).

Fixed by mixing in real crypto.getRandomValues()-sourced entropy from the
host page (new sim_instance_entropy(), read the same way simInstanceTag
already is) into both SimRNG::begin() and the twin seeding pattern in
SimRadio::getRngSeed(). Applies to every sim instance uniformly (hero/B/R
all get proper per-session entropy now), not just the new relay path --
strictly an improvement with no compatibility concern, since identity
generation only ever runs once per fresh/empty IDBFS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018iubftDmKNWmkNnhJRz8UH

variants/sim/SimInstance.h

@@ -51,6 +51,35 @@ inline uint32_t sim_instance_salt() {
     return h >>> 0;
   });
 }
+
+// meshcore-solo-site's WebSocket relay bridge (Phase 3) exposed a real gap
+// in sim_instance_salt() alone: it's a pure function of the simInstanceTag
+// STRING ('hero'/'B'/'R'), so it's only ever useful for telling apart
+// same-tab instances that use different tags -- it's the SAME value every
+// time for two genuinely different browser tabs/machines that both boot a
+// 'hero' instance, which is exactly the new cross-visitor case. Combined
+// with SimRNG::begin()'s other two seed ingredients -- time(NULL) (1-second
+// resolution: two real visitors loading the page in the same second collide
+// outright) and `(uintptr_t)this` (a WASM linear-memory address, which is
+// fully deterministic across independent boots of the same binary doing the
+// same allocation sequence -- there's no ASLR inside a wasm sandbox, so this
+// contributes zero actual entropy, not "usually" different) -- two distinct
+// real visitors landing on the exact same wall-clock second reliably
+// produced byte-identical generated Ed25519 identities (confirmed while
+// testing the relay bridge: two independent, freshly-IDBFS browser contexts
+// launched together produced provably identical advert packets end to end).
+// A host page now passes one genuinely random value from the one place that
+// actually has real entropy per browser session -- crypto.getRandomValues()
+// -- as `simEntropy` on the Module config object (see meshcore-solo-site's
+// bootInstance()/bootRepeater()), read back here the same way
+// simInstanceTag already is. Defaults to 0 (this function's old, sole
+// behavior) if a host page doesn't set it, so nothing else changes.
+inline uint32_t sim_instance_entropy() {
+  return (uint32_t)EM_ASM_INT({
+    return (typeof Module !== 'undefined' && Module['simEntropy']) ? (Module['simEntropy'] >>> 0) : 0;
+  });
+}
 #else
 inline uint32_t sim_instance_salt() { return 0; }
+inline uint32_t sim_instance_entropy() { return 0; }
 #endif