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/lower/coerce.zig

@@ -647,6 +647,16 @@ pub fn coerceMode(self: *Lowering, val: Ref, src_ty: TypeId, dst_ty: TypeId, mod
             const unwrapped = self.builder.emit(.{ .optional_unwrap = .{ .operand = val } }, child_ty);
             return self.coerceMode(unwrapped, child_ty, dst_ty, mode);
         },
+        // Optional → bool: there is no implicit presence-test coercion. The
+        // old unwrap-then-narrow ladder silently produced `false` for every
+        // optional (issue 0169). Reject with a fix-it pointing at `!= null`.
+        .optional_to_bool_reject => {
+            if (self.diagnostics) |d| {
+                const cs = self.builder.current_span;
+                d.addFmt(.err, ast.Span{ .start = cs.start, .end = cs.end }, "cannot use a value of type '{s}' where 'bool' is expected; test presence explicitly with '!= null'", .{self.formatTypeName(src_ty)});
+            }
+            return val;
+        },
         // string → cstring: ONLY a string LITERAL coerces implicitly — its
         // bytes are a terminated constant (Odin's literal blessing). Any
         // other string may be an unterminated view, so it must materialize