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/w6c/main.combined.ww

@@ -74,18 +74,18 @@ export fn dup2(oldfd: i32, newfd: i32) i32 = {
 	return syscall2(SYS_DUP2, oldfd: i64, newfd: i64): i32;
 };
 
-// Fallible wrappers. The error variant is a plain str (Plan 9 errstr
-// model, see lib/errors); the sum type makes success/failure explicit
-// without overloading length-zero.
-export fn tryread(fd: i32, buf: *u8, n: u64) (i64 | str) = {
+// Fallible wrappers. The error variant is `oserror` (an i64 carrying
+// -errno). The sum type makes success/failure explicit and lets
+// callers `?` the result up the stack.
+export fn tryread(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 	let r: i64 = read(fd, buf, n);
-	if (r < 0) { return "read failed"; };
+	if (r < 0) { return r: oserror; };
 	return r;
 };
 
-export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | str) = {
+export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 	let r: i64 = write(fd, buf, n);
-	if (r < 0) { return "write failed"; };
+	if (r < 0) { return r: oserror; };
 	return r;
 };
 
@@ -97,9 +97,9 @@ export fn open(path: *u8, flags: i32, mode: i32) i32 = {
 	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | str) = {
+export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
-	if (fd < 0) { return "open failed"; };
+	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
 };
 
@@ -112,9 +112,10 @@ export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
 
 // oserror — the underlying errno from a failed syscall, as a
 // negative i64 (Linux's int convention; e.g. -2 = ENOENT). The
-// NAMED-i64 alias makes it a distinct variant tag from a "good"
-// i64 byte count. Hare's analogue is errors::errno.
-export type oserror = i64;
+// `!`-flagged alias makes ?-propagation pick this variant as the
+// error half of any (T | oserror) shape. Hare's analogue is
+// errors::errno carried inside io::error.
+export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {