elreal Phase 6: negation, multiplication, division

[Ravenwater]

Parent epic: #923
Depends on: Phase 5 (summation)

Goal

Round out the field operations. With addition and summation in place, the remaining algebraic operations are derived constructions.

Primitives

• `negate(ZBCL x) -> ZBCL`: flip the sign on each block. Trivially preserves 0-overlap. (Section 4.2.4)
• `mul(ZBCL x, ZBCL y) -> ZBCL`: distribute over co-lists using `twoMult` for partial products, gather with `sum`. (Section 4.2.5)
• `div(ZBCL x, ZBCL y) -> ZBCL`: long-division-style refinement using `twoDiv`. (Section 4.2.6)

Acceptance criteria

• `negate`, `mul`, `div` implemented per dissertation 4.2.4 / 4.2.5 / 4.2.6
• Validity proofs sketched inline (value preservation + 0-overlap preservation)
• Algebraic laws: `-(-x) = x`, `x * 1 = x`, `x / x = 1` (modulo zero), `x * y == y * x`, distributivity `x * (y + z) == (xy) + (xz)` -- all checked stream-wise to depth N
• Edge cases: zero, signed zero, very small / very large operands, blocks with extreme `exp_offset`
• Division by zero behaviour (under elreal's exception macro guard, consistent with existing types)
• Tests parameterised over FpType in `{double, float, bfloat16}`
• Tests live in `elastic/elreal/arithmetic/`

Out of scope

• sqrt and transcendentals (Phase 7)
• Optimised multiplication algorithms (Karatsuba etc.) -- defer

Reference

McCleeary 2019 dissertation, Sections 4.2.4 (negation), 4.2.5 (multiplication), 4.2.6 (division).