Add Coerce support for Accessors/Invokers

[Rongmario]

• Allows @Coerce on @Accessor and @Invoker methods so mixin authors can use supertypes (Object, ifaces) when the actual target field/method type is inaccessible.
• When coerced, target resolution falls back to name-only matching (descriptor is omitted) so the inaccessible type doesn't need to appear in the accessor signature.
• For field setters with coercion, a CHECKCAST is emitted before PUTFIELD to narrow the supertype parameter back to the concrete field type.
• Annotation processor validation is relaxed accordingly. Coerced accessors/invokers no longer trigger type-mismatch errors.