lib/os+selfhost: *u8→str path migration (#23)

Path-shaped entrypoints now take str: open, tryopen, access, remove,
mkdir, rmdir, mkdirs, stat, lstat, exists, execve (path arg only).
Each cites its Hare source (ref/hare/os/*.ha, ref/hare/sys/+linux/
*.ha).

New internal kpath(str) *u8 copies into module-level pathbuf: [4096]u8
and NUL-terminates; mirrors ref/hare/sys/+linux/syscalls.ha:25,53.
Non-reentrant — graduates with thread story. mkdirs flattens to one
kpath at entry then walks pathbuf invoking raw SYS_mkdir to avoid
nested kpath clobber.

One Hare divergence at kpath: ships *u8 with nil ENAMETOOLONG sentinel
instead of (*const u8 | errno). Reason: wwstage over-allocates
1-word-payload tagged returns to 24B (cstage emits 16B); filed as
follow-up. Repro at .ai/probe_tagged_return_pointer_payload.ww;
graduates when fix lands.

Each selfhost cmd grew a private pathstr(*u8) str (cstrlen + bs) for
remaining *u8 path sites; w6l shares via obj.ww. Probe 7 in smoke
updated.

Tests 975/976/981 cover migrated entrypoints; 976 extended with two
ENAMETOOLONG rows (-36 for stat, false for exists).

selfhost/test/smoke.ww

@@ -154,7 +154,7 @@ export fn main() i32 = {
 	// that step. RDONLY is 0; passing the literal keeps the call
 	// site standalone-compilable to byte-identical asm on both
 	// compilers.
-	let fd: i32 = os.open(path.ptr, 0, 0i32);
+	let fd: i32 = os.open(path, 0, 0i32);
 	if (fd < 0) { return 21; };
 	let rbuf: [128]u8;
 	// Use raw os.read here (single syscall, plain i64) instead of