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/wcc/check.ww

@@ -108,6 +108,28 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			let s: *sym = scopelookup(c.cur, nm);
+			// `pkg.Type` — strip the last dot prefix and look up
+			// the leaf if `pkg` is a use-imported name. Mirrors
+			// cmd/wcc/check.c resolve_typename.
+			if (s == nil) {
+				let dot: i32 = nm.len - 1;
+				for (dot >= 0) {
+					if (nm[dot] == 46u8) { break; };
+					dot -= 1;
+				};
+				if (dot > 0) {
+					let head: str;
+					head.ptr = nm.ptr;
+					head.len = dot;
+					let m: *sym = scopelookup(c.cur, head);
+					if (m != nil) {
+						let leaf: str;
+						leaf.ptr = nm.ptr + (dot + 1): u64;
+						leaf.len = nm.len - (dot + 1);
+						s = scopelookup(c.cur, leaf);
+					};
+				};
+			};
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {