strconv: graduate to (T | invalid | overflow); add void expression

`type invalid = i32` (payload: byte index of first bad rune; mirrors
Hare's strconv::invalid = !size) and `type overflow = void` (Hare's
overflow = !void). stoi64/stou64 now return these instead of the
str-error placeholder. atoi64 dropped — lib/CLAUDE.md says graduate
in one go, don't keep both shapes around.

To produce the void variant payload, `void` is now a real
expression literal (TK_VOID kw, N_VOIDLIT). It evaluates to ty_void;
codegen emits MOVQ $0, AX. Both kinds are appended at the tail of
their enums to keep prior numeric values byte-stable for the
wwdump-diff fixtures.

check_file reorder: USE declarations are now installed in pass 1
alongside the type-decl placeholders so dotted type references
(`strconv.invalid` from a typedecl body) resolve. DEF/FN/LET silently
overwrite a USE-occupied slot — matches the old behavior where USE
silently no-op'd when a same-name fn/def existed (the conflict
manifested in selfhost main.combined.ww at `use parse;` colliding
with `export fn parse(a)`).

selfhost mirror: lib/ww/lex/tok.ww kwtab+name; lib/ww/ast.ww
N_VOIDLIT def+print; lib/ww/parse/{expr,parse}.ww TK_VOID handling;
selfhost/cmd/wcc/cgenexpr.ww N_VOIDLIT codegen.

lib/ww/parse/expr.ww

@@ -52,6 +52,10 @@ fn parseprimary(p: *parser) *node = {
 		advance(p);
 		return newnode(p.a, N_NIL, pf, pl, pc);
 	};
+	if (p.curkind == TK_VOID) {
+		advance(p);
+		return newnode(p.a, N_VOIDLIT, pf, pl, pc);
+	};
 	if (p.curkind == TK_UNDER) {
 		// Bare `_` — valid only as a discard lvalue. Emit an N_IDENT
 		// with empty str; the checker rejects it outside lvalue

lib/ww/parse/parse.ww

@@ -164,6 +164,15 @@ fn parsetype(p: *parser) *node = {
 		return n;
 	};
 
+	if (p.curkind == TK_VOID) {
+		// `void` keyword in type-expr context — emit as N_TNAME so
+		// resolution treats it like any other primitive name.
+		let n: *node = newnode(p.a, N_TNAME, pf, pl, pc);
+		n.str = "void";
+		advance(p);
+		return n;
+	};
+
 	if (p.curkind == TK_IDENT) {
 		let n: *node = newnode(p.a, N_TNAME, pf, pl, pc);
 		n.str = p.curtext;