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.

examples/0173-types-int-literal-default-s64.sx

@@ -0,0 +1,42 @@
+// Integer literals default to s64 regardless of context: an unannotated
+// `x := <int literal>` local stays s64 even inside a function whose return
+// type is a narrower integer (the implicit-return target must not type the
+// body's declarations), and a large literal initializer keeps its value.
+// Also covers destructure decls (`a, b := ...`), which share the same rule.
+// Regression (issue 0111): these locals adopted the enclosing fn's return
+// type (s32/s8), silently wrapping `big := 3000000000` to -1294967296.
+
+#import "modules/std.sx";
+
+f :: () -> s32 {
+    x := 0;
+    print("f.x: {}\n", type_name(type_of(x)));
+    0
+}
+
+g :: () -> s8 {
+    x := 0;
+    print("g.x: {}\n", type_name(type_of(x)));
+    0
+}
+
+big_host :: () -> s32 {
+    big := 3000000000;
+    print("big: {} = {}\n", type_name(type_of(big)), big);
+    0
+}
+
+d_host :: () -> s32 {
+    a, b := (1, 2);
+    print("a: {} b: {}\n", type_name(type_of(a)), type_name(type_of(b)));
+    0
+}
+
+main :: () {
+    f();
+    g();
+    big_host();
+    d_host();
+    x := 0;
+    print("main.x: {}\n", type_name(type_of(x)));
+}

examples/expected/0173-types-int-literal-default-s64.exit

@@ -0,0 +1 @@
+0

examples/expected/0173-types-int-literal-default-s64.stderr

@@ -0,0 +1 @@
+

examples/expected/0173-types-int-literal-default-s64.stdout

@@ -0,0 +1,5 @@
+f.x: s64
+g.x: s64
+big: s64 = 3000000000
+a: s64 b: s64
+main.x: s64