os: graduate O_*, SEEK_* defs to flag and whence enums

Mirrors Hare's `fs::flag` and `io::whence`:

    export type flag = enum i32 {
            RDONLY  = 0,
            WRONLY  = 1,
            RDWR    = 2,
            CREATE  = 64,    // 0o100
            TRUNC   = 512,   // 0o1000
    };

    export type whence = enum i32 { SET = 0, CUR = 1, END = 2 };

open/tryopen/lseek signatures take the enum types (`flags: flag`,
`w: whence`) so callers get type-checked: `os.open(p, os.flag.RDONLY,
0)` is the correct shape, and `os.flag.WRONLY | os.flag.CREATE |
os.flag.TRUNC` typechecks as a `flag` via the same-named-type rule.

Callers in selfhost/cmd/{ww,w6c,w6a,w6l,wwdump} updated from
`os.O_RDONLY` etc. to `os.flag.RDONLY`. SYS_* syscall numbers kept
as `def` for now (internal-only, ABI surface, no Hare analogue in
this scope).

selfhost/test/smoke.ww keeps its standalone-compile property by
using a numeric literal (`0`, RDONLY's value) for the open flags
arg — probe 6 in 990_selfhost compiles smoke.ww with no `use`
expansion, so cross-module type refs like `os.flag.RDONLY` can't
resolve there. Untyped 0 → flag via type_isnum.

selfhost/cmd/wwdump/main.ww

@@ -74,7 +74,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 2;
 	};
 
-	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.O_RDONLY, 0i32);
+	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.flag.RDONLY, 0i32);
 	let fd: i32 = -1;
 	match (fdorerr) {
 		case let v: i32 => fd = v;