os: graduate tryopen/trywrite/tryread to (T | oserror)

Final piece of the os module graduation: the three try* wrappers
move off the (T | str) placeholder shape. `oserror` becomes a real
Hare-style error type (`!i64` instead of plain `i64`), so it's
picked up by ?-propagation as the error half without callers having
to name it.

tryread:  (i64 | oserror)   was (i64 | str)
trywrite: (i64 | oserror)   was (i64 | str)
tryopen:  (i32 | oserror)   was (i32 | str)

Callsites updated: wwdump uses tryopen; the e2e trywrite probe
matches on os.oserror and validates -EBADF for a bad fd (-9 instead
of the old "write failed" string length).

selfhost/test/smoke.ww switched to raw os.open(2) instead of
os.tryopen for probe 7 — same reason as the os.readall switch in
the prior commit: probe 6 in 990_selfhost compiles smoke.ww
standalone, and cross-module type refs like `os.oserror` don't
resolve in that mode.

selfhost/cmd/wwdump/main.ww

@@ -74,11 +74,11 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 2;
 	};
 
-	let fdorerr: (i32 | str) = os.tryopen(path, os.O_RDONLY, 0i32);
+	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.O_RDONLY, 0i32);
 	let fd: i32 = -1;
 	match (fdorerr) {
 		case let v: i32 => fd = v;
-		case let e: str => {
+		case let e: os.oserror => {
 			os.write(2, "wwdump: cannot open ".ptr, 20u64);
 			os.write(2, path, argstrlen(path): u64);
 			os.write(2, "\n".ptr, 1u64);