Walker: decline extension functions whose receiver has a supertype off the compile classpath #66

#38 added unreachableSupertypeOf and declines member/extension properties whose receiver class has a supertype missing from the consumer's compile classpath (kotlinc cannot build the member scope: 'Cannot access ... which is a supertype of ...'). Extension functions go through the same receiver cast and have the same latent hazard; no jar has hit it yet.

Completion: a plugin unit test with an extension function on a fixture class extending an off-classpath type is declined with a reason naming the missing supertype; :ksp-fixtures:compose and :artifact desktopTest still green.