sema/ir: kill remaining s64 fallbacks (sema Type + getRefType)

- types.Type: add dedicated `unresolved` variant (mirrors ir.TypeId.unresolved)
  with eql/displayName arms; bridgeType maps it to TypeId.unresolved.
- sema.inferExprType + signature/field resolution: every Type.fromTypeExpr /
  fromName / symbol lookup miss and call/field/index fallthrough now yields
  Type.unresolved instead of a fabricated s(64). A variadic `..xs: []T` slice
  element is taken from T, not a guessed "s32". Genuine literal defaults
  (int=>s64, float=>f32, .len=>s64) kept.
- Builder.getRefType: an unlocatable ref (no active function / out-of-range)
  returns .unresolved, not .s64 -- this is the accurate type source the pack
  mono / binop / null-cmp fixes rely on, so it must not fabricate.

236 examples + unit tests (incl sema) green.

src/types.zig

@@ -29,6 +29,11 @@ pub const Type = union(enum) {
     optional_type: OptionalTypeInfo,
     meta_type: MetaTypeInfo,
     tuple_type: TupleTypeInfo,
+    /// Type resolution failed (sema couldn't infer/resolve). A dedicated
+    /// sentinel — never a legitimate type — so callers can't mistake it for a
+    /// real result the way a fabricated `s(64)` would be. Mirrors
+    /// `ir.TypeId.unresolved`.
+    unresolved,
 
     pub const SliceTypeInfo = struct {
         element_name: []const u8,
@@ -84,7 +89,7 @@ pub const Type = union(enum) {
         return switch (self) {
             .signed => |w| w == other.signed,
             .unsigned => |w| w == other.unsigned,
-            .f32, .f64, .void_type, .boolean, .string_type, .any_type, .usize_type, .isize_type => true,
+            .f32, .f64, .void_type, .boolean, .string_type, .any_type, .usize_type, .isize_type, .unresolved => true,
             .enum_type => |n| std.mem.eql(u8, n, other.enum_type),
             .struct_type => |n| std.mem.eql(u8, n, other.struct_type),
             .union_type => |n| std.mem.eql(u8, n, other.union_type),
@@ -555,6 +560,7 @@ pub const Type = union(enum) {
             .any_type => "Any",
             .usize_type => "usize",
             .isize_type => "isize",
+            .unresolved => "<unresolved>",
             .enum_type => |name| name,
             .struct_type => |name| name,
             .union_type => |name| name,