fix: reject implicit ?T -> bool coercion instead of silent false (issue 0169)

The Optional->Concrete unwrap classify rule treated ?i64 -> bool as
unwrap+narrow (both builtin), silently yielding false for every optional
(present or null). specs.md defines no implicit optional->bool
conversion. Reject it: conversions.zig adds an optional_to_bool_reject
plan (dst == bool, child != bool); coerce.zig emits a located diagnostic
suggesting '!= null'. Covers arg/field-init/return via the shared
coerceMode. The if-opt presence test (issue 0164) is a separate path,
untouched.

Regression: examples/diagnostics/1199-diagnostics-optional-to-bool.sx +
conversions.test.zig unit test. Verified by 3 adversarial reviews, suite
789/0. Filed adjacent issue 0179 (whole implicit ?T->concrete unwrap
family silently miscompiles a null optional; design-touching).

src/ir/conversions.zig

@@ -34,6 +34,7 @@ pub const CoercionResolver = struct {
         closure_to_fn_reject, // closure value → bare fn-ptr (diagnostic, returns operand)
         tuple_elementwise, // (A,B) → (C,D), same arity
         optional_unwrap, // ?T → concrete (narrowing)
+        optional_to_bool_reject, // ?T → bool (no presence-test coercion; diagnostic)
         void_to_optional, // void (null literal) → ?T
         optional_wrap, // concrete → ?T
         erase_protocol, // concrete → protocol value
@@ -99,6 +100,17 @@ pub const CoercionResolver = struct {
             const src_info = self.l.module.types.get(src_ty);
             if (src_info == .optional) {
                 const child_ty = src_info.optional.child;
+                // `?T → bool` is NOT a presence test. The unwrap-then-narrow
+                // ladder below would extract the payload and narrow it to `i1`,
+                // which silently yields `false` for every optional (issue 0169).
+                // There is no implicit optional→bool coercion in the language
+                // (only `T → ?T` wrapping and flow-sensitive narrowing); a bool
+                // position wants an explicit presence test. Reject loudly unless
+                // the payload is itself a bool (`?bool → bool` is a genuine
+                // unwrap of a bool payload, handled by the same arm below).
+                if (dst_ty == .bool and child_ty != .bool) {
+                    return .optional_to_bool_reject;
+                }
                 if (child_ty == dst_ty or (dst_ty.isBuiltin() and child_ty.isBuiltin())) {
                     return .optional_unwrap;
                 }