os: graduate filesize/readall/writeall to (i64 | oserror)

`type oserror = i64` carries -errno (Hare's errors::errno-shaped
named-i64). The three convenience wrappers move off the i64 = -1
sentinel and onto the tagged-union surface.

Callers updated across the selfhost (wwdump, w6c, w6a, w6l, ww
driver). The slurp paths in w6c/w6a/w6l/wwdump now match on the
filesize and readall results; the ELF-emitting writeall sites in
w6a/obj.ww are wrapped through two small local helpers (`wrn` for
"wrote N bytes ok?", `wrdrop` for fire-and-forget) so the existing
11-callsite write loop stays readable.

selfhost/test/smoke.ww kept using raw os.read instead of
os.readall: the 990 cgen-match probe compiles smoke.ww standalone
(no `use` expansion), and cross-module type references like
`os.oserror` can't be resolved in that mode.

Two selfhost-side gaps surfaced and got plugged:
- lib/ww/parse/parse.ww parsetype now collapses dotted type names
  (`pkg.Type` → single N_TNAME with the joined string), mirroring C
  parsetype's dotted-path loop. Local `joindotted` helper because
  there's no arena-based string-concat in the selfhost lib yet.
- selfhost/cmd/wcc/check.ww name-resolver applies the dotted-prefix
  rule from cmd/wcc/check.c's resolve_typename: split at the last
  dot, look up the head as a `use` import, then the leaf as a type.

lib/os/os.ww

@@ -109,25 +109,29 @@ export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
 	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
 };
 
-// filesize — convenience: returns the byte length of an open fd by
-// seeking to the end and back. -1 on error.
-export fn filesize(fd: i32) i64 = {
+// oserror — the underlying errno from a failed syscall, as a
+// negative i64 (Linux's int convention; e.g. -2 = ENOENT). The
+// NAMED-i64 alias makes it a distinct variant tag from a "good"
+// i64 byte count. Hare's analogue is errors::errno.
+export type oserror = i64;
+
+// filesize — byte length of an open fd via lseek-to-end-and-back.
+export fn filesize(fd: i32) (i64 | oserror) = {
 	let end: i64 = lseek(fd, 0i64, SEEK_END);
-	if (end < 0) { return -1i64; };
+	if (end < 0) { return end: oserror; };
 	let r: i64 = lseek(fd, 0i64, SEEK_SET);
-	if (r < 0) { return -1i64; };
+	if (r < 0) { return r: oserror; };
 	return end;
 };
 
 // readall — keep reading until `n` bytes have arrived or the fd
-// closes early. Returns bytes read (0..=n) or -1 on read error.
-// Hare name (io::readall); the buffer is caller-supplied, matching
-// the Plan 9 subset convention.
-export fn readall(fd: i32, buf: *u8, n: u64) i64 = {
+// closes early. Hare name (io::readall); the buffer is caller-
+// supplied, matching the Plan 9 subset convention.
+export fn readall(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 	let got: u64 = 0u64;
 	for (got < n) {
 		let r: i64 = read(fd, buf + got, n - got);
-		if (r < 0) { return -1i64; };
+		if (r < 0) { return r: oserror; };
 		if (r == 0) { return got: i64; };	// short read: caller decides
 		got += r: u64;
 	};
@@ -135,13 +139,12 @@ export fn readall(fd: i32, buf: *u8, n: u64) i64 = {
 };
 
 // writeall — keep writing until `n` bytes have been accepted or the
-// fd refuses progress. Returns bytes written or -1. Hare name
-// (io::writeall).
-export fn writeall(fd: i32, buf: *u8, n: u64) i64 = {
+// fd refuses progress. Hare name (io::writeall).
+export fn writeall(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 	let sent: u64 = 0u64;
 	for (sent < n) {
 		let r: i64 = write(fd, buf + sent, n - sent);
-		if (r < 0) { return -1i64; };
+		if (r < 0) { return r: oserror; };
 		if (r == 0) { return sent: i64; };
 		sent += r: u64;
 	};