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/test/smoke.ww

@@ -159,7 +159,11 @@ export fn main() i32 = {
 		case let e: str => return 21;
 	};
 	let rbuf: [128]u8;
-	let n: i64 = os.readall(fd, rbuf.ptr, 128u64);
+	// Use raw os.read here (single syscall, plain i64) instead of
+	// os.readall: the 990 cgen-match probe compiles smoke.ww
+	// standalone without `use os;` expansion, so cross-module type
+	// references like `os.oserror` can't be resolved.
+	let n: i64 = os.read(fd, rbuf.ptr, 128u64);
 	os.close(fd);
 	if (n <= 0i64) { return 22; };