fix(wasi-runtime): end exit and park without running the program

runtime.js implemented proc_exit by throwing a JavaScript error through
the wasm stack, and the switch to -fwasm-exceptions made that error
catchable: every cleanup pad, catch (...) and noexcept terminate pad
catches a foreign exception. An exit therefore ran the destructors of
every frame it left, a catch (...) swallowed it and carried on, and
under a noexcept frame it ended in std::terminate. Crafter.Graphics
calls std::_Exit(0) in Window::StartSync to hand the thread to
requestAnimationFrame; once the exit's destructors had torn down
main's locals, every later frame read destroyed state.

proc_exit is now a function of a small helper module, bound directly
as the program's import, that reports to the runtime and then executes
unreachable. A trap is the one thing wasm code cannot catch, and raised
in wasm rather than from JS it does not depend on each engine keeping
a trap uncatchable when it passes back through a JS frame. Nothing of
the program runs on the way out and its shadow stack stays as it was,
which is what exit and _Exit require. window.crafter_wasi.exitCode
holds the status, also after a return from main, so that bridges which
call into the program on their own can stop.

Handing the thread to the browser for good is a separate import,
crafter_runtime.park. It ends the call the same way but leaves the
program alive, and main's locals valid for the calls that follow, so
StartSync keeps its contract that no caller code after it runs. Until
Crafter.Graphics moves to park, its _Exit behaves as before minus the
destructors: the runtime records an exit but cannot stop a bridge that
calls exports itself.

A WASI reactor build, where main returns and state lives in statics,
was the alternative. It breaks StartSync's promise never to return,
which the native targets rely on. JSPI would keep the whole stack
alive, but Safari does not have it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

interfaces/Crafter.Build-Clang.cppm

@@ -737,7 +737,11 @@ export namespace Crafter {
     // open it). Opt-in: WASI builds destined for wasmtime/wasmer don't need
     // this and shouldn't carry the extra files. Call from project.cpp after
     // outputName is set; index.html is generated against the current
-    // outputName so renaming the binary later requires another call.
+    // outputName so renaming the binary later requires another call. In that
+    // runtime exit and _Exit end the program without running any more of it;
+    // a program that hands the thread to the browser and keeps serving calls
+    // parks instead, through the crafter_runtime.park import (runtime.js,
+    // PROGRAM_END).
     void EnableWasiBrowserRuntime(Configuration& cfg);
 
     // Register the relaxed-SIMD codegen variant on cfg. Builds an additional