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/cmd/w6c/main.ww

@@ -39,8 +39,18 @@ fn cstrlen(p: *u8) u64 = {
 	return n;
 };
 
+// pathstr — view a NUL-terminated *u8 as a str. lib/os entrypoints
+// take str post-task-#23; this bridges call sites that still hold
+// C-string paths (argv entries, arena-allocated buffers).
+fn pathstr(p: *u8) str = {
+	let r: str;
+	r.ptr = p;
+	r.len = cstrlen(p): i32;
+	return r;
+};
+
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
+	let fd: i32 = os.open(pathstr(path), os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -106,7 +116,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	// cgen.ww writes directly to fd 1; dup2 lets us reuse it without
 	// threading a file descriptor through the emit helpers.
 	if (out != nil) {
-		let ofd: i32 = os.open(out,
+		let ofd: i32 = os.open(pathstr(out),
 		    os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 		if (ofd < 0) {
 			os.write(2, "w6c: cannot open output\n".ptr, 23u64);