Native-subscribe isolation of an unknown symbol, LastSuccessfulRead advance
from pushes alone, a WORD bit write flipping exactly one bit (bit 16
refused), a whole-array REAL write round-trip (length mismatch refused) and a
STRING(80) write round-trip (overflow refused). Every writing case restores
the seed it touched. Skipped without the TwinCAT XAR fixture.
Refs #739
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty