fix(0111): unannotated decl literals no longer adopt the fn return type

lowerVarDecl (unannotated) and lowerDestructureDecl now clear target_type
around the initializer lowering: a declaration without annotation provides
no target, so int/float literals take their spec defaults (s64/f64) instead
of the enclosing function's implicit-return type (x := 0 in a -> s8 fn was
s8; big := 3000000000 in -> s32 silently wrapped to -1294967296).

Regression: examples/0173-types-int-literal-default-s64.sx. The remaining
explicit-annotation wrap (x : s8 = 300) is filed as issue 0112.

src/ir/lower/stmt.zig

@@ -337,9 +337,15 @@ pub fn lowerVarDecl(self: *Lowering, vd: *const ast.VarDecl) void {
         // This is critical for generic calls where the return type is only
         // known after monomorphization.
         const saved_fbv = self.force_block_value;
+        const saved_target = self.target_type;
         self.force_block_value = true;
+        // An unannotated decl provides no target type: clear the ambient one
+        // (the enclosing fn's implicit-return target) so literal initializers
+        // take their spec defaults (s64/f64) instead of adopting it.
+        self.target_type = null;
         const ref = self.lowerExpr(val);
         self.force_block_value = saved_fbv;
+        self.target_type = saved_target;
         const ty = self.builder.getRefType(ref);
         const slot = self.builder.alloca(ty);
         self.builder.store(slot, ref);
@@ -1189,9 +1195,14 @@ pub fn lowerMultiAssign(self: *Lowering, ma: *const ast.MultiAssign) void {
 pub fn lowerDestructureDecl(self: *Lowering, dd: *const ast.DestructureDecl) void {
     // Lower the RHS expression (must produce a tuple)
     const saved_fbv = self.force_block_value;
+    const saved_target = self.target_type;
     self.force_block_value = true;
+    // Same as the unannotated var-decl path: the destructure declares new
+    // bindings, so the ambient target type must not type the RHS literals.
+    self.target_type = null;
     const ref = self.lowerExpr(dd.value);
     self.force_block_value = saved_fbv;
+    self.target_type = saved_target;
     const ty = self.builder.getRefType(ref);
 
     // Get tuple field info