Update EVAL.md: progress log, lessons learned, corrected next steps

Records what's actually landed (pawns, bbOccupied, dispatch loop,
knight, bishop -- commits 57502d6/2ce3570/6e86450/7e3e6b6/de3f366),
corrects section 4's per-piece #define toggle strategy (not what was
actually used -- direct incremental rewrites with transitional
helpers instead), and documents methodology worth repeating:

- One piece type at a time, each landed and verified before the next
  starts, learned the hard way after a combined knight+bishop+rook+
  queen+king attempt produced a real bug that was hard to isolate
  with five things changed at once and had to be reverted.
- Keep writing DEBUG asserts against the still-live rgSquare mailbox
  representation as independent ground truth for as long as it
  exists -- this is specifically what caught bishop's x-ray-chain
  behavior gap during routine smoke testing.
- Verification bar changes once a step ships a deliberate behavior
  change (bishop's x-ray simplification): full ecm_ringers solve
  parity + bounded node-count deltas, not byte-identical counts.
- This effort errs on the side of speed over exact fidelity,
  explicitly -- a real, recorded change from the original plan's
  "must be byte-identical" bar.
- What's next: rook, then queen, then _EvalKing/_WhoControlsSquareFast,
  culminating in deleting bvAttacks/ATTACK_BITV/the c|8 mechanism
  entirely once nothing writes it anymore. Transitional helpers get
  rewritten incrementally as each piece converts, not left to
  accumulate special cases.
- Pawn hash hit/miss cost measurements (90 vs 1741 cycles) and the
  EVAL_TIME per-term breakdown methodology, both worth reusing on
  rook/queen once they land.
- Noted intent to revisit the non-mobility scoring terms in
  _EvalKnight/_EvalBishop/_EvalRook/_EvalQueen once the attack-bits
  work is done -- separate, later pass, not scoped further yet.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01P6g6iF6mD1Hau6nCZCzwYj

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