Route pypal through wasi imports so NilPy can do file I/O on wasm32

Where this starts from

[[bug-n-the-nilpy-pal-issues-raw-syscalls-so-every-file-body-traps-on-wasm32]] settled the fork two ways: the Pascal RTL got REAL wasi (the backend already existed; only the routing was missing), and pypal got the ESP precedent — a defined failure. That was the right split for unblocking compilation, and it leaves NilPy on wasm32 able to run and unable to touch a file.

PyPalSys is now the single site. On a target without a syscall table its body is -1 and no syscall instruction is emitted. That single site is also the seam this feature needs — a wasi arm goes there, or beside it.

What wasi preview1 actually has

Real imports, so these are implementable rather than refusable: path_open, fd_read, fd_write, fd_close, fd_seek, fd_filestat_set_size, path_unlink_file, path_rename, path_readlink, fd_readdir, clock_time_get. lib/rtl/platform/wasi/platform_backend.pas already binds this set for the Pascal side — read it first; the argument marshalling is the work and it is done once there.

ppoll is the one that does not map cleanly (wasi has poll_oneoff with a different subscription model), and getcwd is a preopen-table question rather than a call. Refusing those two and implementing the rest is a complete product.

The thing to be careful about

wasi paths are relative to a PREOPENED directory descriptor; there is no ambient filesystem root. So PyPalOpen(path, flags, mode) cannot be a one-to-one translation — it needs the preopen lookup the Pascal backend already does. Do not invent a second one. If that lookup wants to be shared, share it deliberately rather than by copying, and note that the per-language PAL duplication is otherwise ACCEPTED on purpose (decide-runtime-primitive-layering).

How you will know it works

A NilPy program cannot RUN on wasm32 yet for unrelated codegen reasons (IR_ZERO_SYM and friends), so this cannot be end-to-end tested today. Until that clears, the honest gate is a compile plus the wasi import section containing path_open — and say so, rather than reporting a green that only means the module was built.