feat(pack): asset packs, so a built game is a thing you can hand someone

A Ludic game opened its assets by a path relative to the working directory, so
`build/mygame` ran only from the project root and there was nothing to give
anyone but "the binary, and also this whole tree". A game is not one file, but
shipping it has to be.

`ludic pack` writes a .lpak: a header, a name-sorted entry table, a name heap
and the blobs, 16-byte aligned. Entries are stored rather than compressed - PNG,
JPEG and glTF binary arrive compressed already, and a decompressor on the load
path would spend CPU to make the file no smaller.

The reader is spliced into the compiler at the one place every asset in a Ludic
program comes through: file_open. gltf_load, tex_load, Audio.load, Fs.read_text
and the renderer's own shader loads all bottom out there, so routing it through
@lp_pak_open reaches every one of them without any of them knowing. A read of a
packed path becomes an fmemopen over the mapped bytes; everything else is the
fopen it always was. Fs.exists and Fs.size consult the packs too, so a game that
guards a load with Fs.exists keeps finding its assets once they are packed.

The pack is mmap'd rather than read: 165 MB of textures costs one syscall at
boot and pages in only what is touched. @lp_pak_boot runs from
@llvm.global_ctors, before main, so a pack is mounted before the game's first
line - and it reads packs.index, a plain list, so the mount order is explicit
and a later pack shadows an earlier one.

Without a packs.index nothing mounts and every open goes to the filesystem
exactly as before, which is every `ludic run` during development. Packing is a
shipping step and is invisible until you ship.

The compiler reproduces itself byte-exactly and the C-free bootstrap from the
seed still holds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/ludic-cli/selfhost.ludic

@@ -37,6 +37,7 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/backend/stdlib/emit_os.ludic")
   push(f, "selfhost/backend/stdlib/emit_unicode.ludic")
   push(f, "selfhost/backend/stdlib/emit_fs.ludic")
+  push(f, "selfhost/backend/stdlib/emit_pak.ludic")
   push(f, "selfhost/backend/stdlib/emit_list.ludic")
   push(f, "selfhost/backend/stdlib/emit_ease.ludic")
   push(f, "selfhost/backend/stdlib/emit_anim.ludic")