Drop CountKingSafetyDefects from board-representation migration plan

Descope CountKingSafetyDefects entirely, per discussion after landing
GetAttacks's half of section 3: the two functions no longer share
enough to justify one plan. CountKingSafetyDefects (eval.c:2325) turns
out to do no ray-walk/blocker check at all -- it's an unblocked
CHECK_VECTOR proximity heuristic, not a true attack query -- so
_WhoAttacksSquareBB's blocker-aware result isn't a value-identical
drop-in for it; making it bitboard-backed would be a real behavior
change (needing eval re-tuning/re-gating), not a reimplementation, and
a materially different, riskier project than this one.

eval.c is untouched. If CountKingSafetyDefects work happens later, it
should be a new, separate migration document starting from its actual
(unblocked heuristic) behavior, not a resumption of this one.

Also updates section 4-7 status notes to reflect what's actually done
vs. still blocked on section 6's toggle (GetAttacks alone, no longer
entangled with a king-safety timeline).

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Jntky4yGUTyQVaGCXms4F2

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