M1: add missing (object) cast to UInt64 arm of DecodeScalarBlock switch expression,
matching the Int64 arm style and the comment that each arm is boxed explicitly.
M2: short-circuit Timer/Counter writes in WriteAsync to BadNotWritable before
WriteOneAsync, so transient equipment-tag refs (Writable=true from parser) return
the same status code as authored tags rejected at init — documented in the docs.
Adds 6 pure unit tests pinning the area-detection precondition the guard relies on.
EncodeScalarBlock Timer/Counter throws remain as the defensive backstop.
Joseph Doherty