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/w6l/out.ww

@@ -90,10 +90,20 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
 	wru64(hdr, 112u64, TEXT_OFF);		// p_align
 
 	// Write [0..0x1000) then .text.
-	let n1: i64 = os.writeall(fd, hdr, TEXT_OFF);
+	let r1: (i64 | os.oserror) = os.writeall(fd, hdr, TEXT_OFF);
+	let n1: i64 = 0i64;
+	match (r1) {
+	case let v: i64 => n1 = v;
+	case let e: os.oserror => return -1;
+	};
 	if (n1 != TEXT_OFF: i64) { return -1; };
 	if (l.textlen > 0u64) {
-		let n2: i64 = os.writeall(fd, l.text, l.textlen);
+		let r2: (i64 | os.oserror) = os.writeall(fd, l.text, l.textlen);
+		let n2: i64 = 0i64;
+		match (r2) {
+		case let v: i64 => n2 = v;
+		case let e: os.oserror => return -1;
+		};
 		if (n2 != l.textlen: i64) { return -1; };
 	};
 	return 0;