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.

selfhost/cmd/w6a/main.ww

@@ -34,12 +34,21 @@ fn cstrlen(p: *u8) u64 = {
 fn slurp(path: *u8) (*u8, u64) = {
 	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
-	let n: i64 = os.filesize(fd);
-	if (n < 0i64) { os.close(fd); return nil, 0u64; };
+	let szr: (i64 | os.oserror) = os.filesize(fd);
+	let n: i64 = 0i64;
+	match (szr) {
+	case let v: i64 => n = v;
+	case let e: os.oserror => { os.close(fd); return nil, 0u64; };
+	};
 	let nz: u64 = n: u64;
 	let buf: *u8 = os.alloc(nz + 1u64): *u8;
-	let got: i64 = os.readall(fd, buf, nz);
+	let rr: (i64 | os.oserror) = os.readall(fd, buf, nz);
 	os.close(fd);
+	let got: i64 = 0i64;
+	match (rr) {
+	case let v: i64 => got = v;
+	case let e: os.oserror => return nil, 0u64;
+	};
 	if (got != n) { return nil, 0u64; };
 	buf[nz] = 0u8;
 	return buf, nz;