lib: add temp + os.mkdir/rmdir/EXCL

temp mirrors Hare's temp: file, named, dir. file() routes through
named() and discards the path (no O_TMPFILE yet). Path randomizer
uses inline SplitMix64 seeded from getpid + O_EXCL retry (Hare uses
crypto::random which we don't ship). named() takes out-pointers for
fd + path — return shape gated on tasks #5 and #11. Caller closes
and removes; no defer in ww.

os gains mkdir, rmdir, flag.EXCL — straight ports of ref/hare/os.
selfhost combined files cascade; 995_self_rebuild byte-identity
holds.

selfhost/cmd/w6a/main.combined.ww

@@ -36,6 +36,8 @@ type nr = enum i64 {
 	EXECVE      = 59,
 	EXIT        = 60,
 	WAIT4       = 61,
+	MKDIR       = 83,
+	RMDIR       = 84,
 	UNLINK      = 87,
 	GETCWD      = 79,
 	GETDENTS64  = 217,
@@ -49,6 +51,7 @@ export type flag = enum i32 {
 	WRONLY  = 1,
 	RDWR    = 2,
 	CREATE  = 64,	// 0x40
+	EXCL    = 128,	// 0x80 — pair with CREATE to fail on existing path
 	TRUNC   = 512,	// 0x200
 };
 
@@ -178,6 +181,19 @@ export fn remove(path: *u8) i32 = {
 	return syscall1(nr.UNLINK, path: i64): i32;
 };
 
+// mkdir — mkdir(2). Path must be NUL-terminated. Mode is the unix
+// permission bitset (e.g. 0o700). Returns 0 on success, negative
+// errno otherwise. Hare name (os::mkdir).
+export fn mkdir(path: *u8, mode: i32) i32 = {
+	return syscall2(nr.MKDIR, path: i64, mode: i64): i32;
+};
+
+// rmdir — rmdir(2). Path must be NUL-terminated. Returns 0 on
+// success, negative errno otherwise. Hare name (os::rmdir).
+export fn rmdir(path: *u8) i32 = {
+	return syscall1(nr.RMDIR, path: i64): i32;
+};
+
 // getpid(2). Used by the driver to mint unique scratch paths.
 export fn getpid() i32 = {
 	return syscall0(nr.GETPID): i32;