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/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