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/ww/parse/parse.ww

@@ -104,6 +104,26 @@ fn expectbindname(p: *parser, into: *str) bool = {
 // Other forms (slice, array, struct, fn, chan, tuple, tagged) will
 // land in subsequent commits.
 
+// joindotted — arena-build "head.tail" for dotted type-name path
+// collapse. Mirrors aprintf in C parser; pulled local to avoid a
+// cross-module dependency.
+fn joindotted(a: *arena, head: str, tail: str) str = {
+	let n: u64 = head.len: u64 + 1u64 + tail.len: u64;
+	let p: *u8 = amalloc(a, n + 1u64): *u8;
+	let i: u64 = 0u64;
+	let j: i32 = 0;
+	for (j < head.len) { p[i] = head[j]; i += 1u64; j += 1; };
+	p[i] = 46u8;	// '.'
+	i += 1u64;
+	j = 0;
+	for (j < tail.len) { p[i] = tail[j]; i += 1u64; j += 1; };
+	p[i] = 0u8;
+	let r: str;
+	r.ptr = p;
+	r.len = n: i32;
+	return r;
+};
+
 fn parsetype(p: *parser) *node = {
 	let pf: str = p.curfile;
 	let pl: i32 = p.curline;
@@ -175,10 +195,17 @@ fn parsetype(p: *parser) *node = {
 
 	if (p.curkind == TK_IDENT) {
 		let n: *node = newnode(p.a, N_TNAME, pf, pl, pc);
-		n.str = p.curtext;
+		let acc: str = p.curtext;
 		advance(p);
-		// Dotted path collapse (pkg.Type) deferred — fixtures don't
-		// need it yet.
+		// Dotted path collapse: pkg.Type → single TNAME with the
+		// joined string. Mirrors C parsetype's loop.
+		for (p.curkind == TK_DOT) {
+			advance(p);
+			if (p.curkind != TK_IDENT) { break; };
+			acc = joindotted(p.a, acc, p.curtext);
+			advance(p);
+		};
+		n.str = acc;
 		return n;
 	};