PrimEntry: exact rational arithmetic (num/den, no truncation).
All intermediate steps stay in ℤ. Single final division by det(A).
Key insight: A × adj(A) = det(A) × I is exact over ℤ.
Q16_16 version has 1-LSB error per entry (demonstrated by #eval).
Witnesses:
Primitive: 1/3 + 1/6 = 32768 (exact 0.5)
Q16_16: div(1,3) + div(1,6) = 32767 (1 LSB error)
Restored det_self_inverse theorem (removed by subagent).
lake build: 3301 jobs, 0 errors
