pattern 9: eq

parameterized by F.

abstract:   eq(a, b) → 0 if a = b (data identity), else 1
canonical:  0 if v_a = v_b else 1

eq compares data identity: atoms by field value, pairs by structural digest (Hemera hash of left/right sub-digests). returns 0 if equal, 1 if not equal (consistent with branch: 0 = take yes-branch). for pairs, the comparison uses the particle digest — structurally equal pairs always produce equal digests.

cost: 1. constraints: 1.