wasm: fix pointer builtins, missing -sASYNCIFY, and add main_loop()

[nbeerbower]

Three independent fixes to the browser story, all verified against
Emscripten 4.0.7 + Node.

1. Raw pointer builtins no longer panic (or fail to link) under WASM.

   builtins_ffi.c was wrapped entirely in #ifndef EMSCRIPTEN, which
   swept up the ptr_offset / ptr_read_* / ptr_write_* / ptr_deref_*
   family living at the bottom of the file. Those need neither dlopen
   nor libffi -- they are address arithmetic and loads/stores over
   linear memory -- so the guard now closes before them.

   Four of them (ptr_offset, ptr_read_i32, ptr_write_i32,
   ptr_deref_i32) had panicking stubs in wasm_shim.c; those are gone.
   The other ten (ptr_read_i8/i16/u8/u16/u32/i64/u64/ptr/f32/f64) had
   no stub at all, so a program touching them failed at wasm-ld with
   "undefined symbol". Both cases now work.

2. --target wasm links with -sASYNCIFY.

   hml_sleep() compiles to emscripten_sleep(), which only exists when
   Asyncify is on. Without the flag Emscripten links a throwing stub,
   so the program dies the first time it sleeps:

   Please compile your program with async support in order to use
   asynchronous operations like emscripten_sleep

   The threaded branch already documented the flag in a comment but
   never passed it. Both branches pass it now.

3. New main_loop(callback, fps) / main_loop_stop() in @stdlib/time.

   Asyncify is not free: it instruments the module so it can unwind and
   rewind the WASM stack at any suspend point. Measured here at ~1.3-1.4x
   on CPU-bound code and +31% .wasm size.

   main_loop() hands the frame callback to the host's own scheduler
   (emscripten_set_main_loop_arg) instead, so a frame loop needs no
   sleep() and the program can be linked with the new --no-asyncify.
   It also gets the timing right: fps 0 means requestAnimationFrame,
   i.e. vsync-locked and throttled in background tabs, where a sleep()
   loop is not frame-locked at all.

   Natively it is a paced loop that returns once stopped. Under
   --target wasm it does not return, since the host owns the event loop
   from that point on -- documented in stdlib/docs/time.md.

Tests: tests/wasm/wasm_pointers.hml and tests/wasm/wasm_main_loop.hml
(both run through emcc + node in the WASM suite), plus a
tests/parity/builtins/main_loop.hml parity test covering frames,
restart, fps pacing, closures and the error paths.

make test 715 passed / 2 failed (both pre-existing on main: no
libwebsockets and no IPv6 in this container), parity 321/321,
test-compiler 54/54, test-contracts 23/23, wasm-test 19 passed.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01FsuZNWvEWeYRDTk3jzNNPL