wcc+selfhost: pkg-qualified enum access (os.whence.CUR)

Driver-side concatenation flattens module names, but enum member
lookup keyed off the exact lhs ident — so `whence.CUR` worked while
`os.whence.CUR` fell through to w6l with `undefined main.whence`.

C side: fold TY_ENUM members in the post-cexpr cascade too, not
just the early SK_TYPE shortcut. The recursive cexpr lands the
inner N_DOT(os, whence) on the named enum type; the outer access
then folds normally.

Selfhost: cgdot now treats `N_IDENT.MEMBER` and `N_DOT.MEMBER` the
same way, keying off the leaf name. enumlookup strips a trailing
`.`-prefix from the lookup key.

Adds e2e test 700: `use os; os.whence.CUR as i32 == 1`.

selfhost/cmd/wcc/cgen.ww

@@ -186,9 +186,22 @@ fn collectenums(c: *cgen, file: *node) void = {
 };
 
 fn enumlookup(c: *cgen, name: str) *enumtype = {
+	// Strip any `pkg.` prefix and key off the leaf — driver-side
+	// concatenation flattens the namespace, so `os.whence` and
+	// `whence` refer to the same registered enum.
+	let leaf: str = name;
+	let i: i32 = name.len - 1;
+	for (i >= 0) {
+		if (name[i] == 46u8) {	// '.'
+			leaf.ptr = name.ptr + (i + 1): u64;
+			leaf.len = name.len - (i + 1);
+			break;
+		};
+		i -= 1;
+	};
 	let e: *enumtype = c.enums;
 	for (e != nil) {
-		if (streq(e.ename, name)) { return e; };
+		if (streq(e.ename, leaf)) { return e; };
 		e = e.etnext;
 	};
 	return nil;