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/ast.ww

@@ -88,8 +88,9 @@ def N_MASSIGN:  i32 = 59;
 // 990_selfhost test diffs astprint against the C side byte-for-byte.
 def N_TYPETEST:   i32 = 60;
 def N_TYPEASSERT: i32 = 61;
+def N_VOIDLIT:    i32 = 62;
 
-def N_LAST:     i32 = 62;
+def N_LAST:     i32 = 63;
 
 // ---- Node -------------------------------------------------------------
 
@@ -190,6 +191,7 @@ fn nkname(k: i32) str = {
 	if (k == N_MASSIGN)   { return "massign"; };
 	if (k == N_TYPETEST)   { return "typetest"; };
 	if (k == N_TYPEASSERT) { return "typeassert"; };
+	if (k == N_VOIDLIT)   { return "voidlit"; };
 	if (k == N_LAST)      { return "last"; };
 	return "?";
 };

lib/ww/lex/tok.ww

@@ -108,9 +108,10 @@ def TK_FATARROW: i32 = 81;
 // Appended at the tail (not grouped with the keyword block) so every
 // pre-existing TK_* value stays unchanged — the 990_selfhost test
 // diffs wwdump output against the C side, byte for byte.
-def TK_IS: i32 = 82;
+def TK_IS:   i32 = 82;
+def TK_VOID: i32 = 83;
 
-def TK_LAST: i32 = 83;
+def TK_LAST: i32 = 84;
 
 // ---- Pos / Tok --------------------------------------------------------
 //
@@ -181,6 +182,7 @@ export fn kwlookup(p: *u8, n: i32) i32 = {
 	if (streqn(p, "true", n))     { return TK_TRUE; };
 	if (streqn(p, "type", n))     { return TK_TYPE; };
 	if (streqn(p, "use", n))      { return TK_USE; };
+	if (streqn(p, "void", n))     { return TK_VOID; };
 	return TK_NONE;
 };
 
@@ -222,6 +224,7 @@ export fn tokname(k: i32) str = {
 	if (k == TK_FALSE)    { return "false"; };
 	if (k == TK_AS)       { return "as"; };
 	if (k == TK_IS)       { return "is"; };
+	if (k == TK_VOID)     { return "void"; };
 	if (k == TK_STATIC)   { return "static"; };
 	if (k == TK_MATCH)    { return "match"; };
 	if (k == TK_CONST)    { return "const"; };

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;