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`.

test/wcc/700_e2e.c

@@ -1100,6 +1100,13 @@ static const struct row rows[] = {
 	  "  let m: mode = mode.R | mode.W;\n"
 	  "  return m as i32;\n"
 	  "};", 3 },
+	/* enum: pkg-qualified access — `os.whence.CUR` resolves through
+	 * SK_USE and folds to the member literal. */
+	{ "// MODULE: os\n"
+	  "type whence = enum { SET, CUR, END };\n"
+	  "// MODULE: main\n"
+	  "use os;\n"
+	  "fn main() i32 = { return os.whence.CUR as i32; };", 1 },
 	{ NULL, 0 }
 };