- att.py: raise core.InvalidPacketError instead of generic ValueError
- smp.py: raise core.InvalidPacketError instead of generic ValueError
- hfp.py: add MAX_BUFFER_SIZE class constant (64KB)
- hfp.py: drop incoming data when it would overflow buffer instead of
truncating, preserving existing partial-packet state
Per review comments on PR #912 by @zxzxwu.