test(hex-poly): cover dense polynomial negation

[kim-em]

Closes #9387.

Add generic DensePoly Int conformance coverage for the public negation operation using the existing typical, zero-edge, and sparse adversarial fixtures. The checks pin coefficientwise output, additive inverse, involution, and agreement between subtraction and adding a negation. Extend the module contract and library SPEC to advertise the operation, properties, and edge cases.

Directive validation: DensePoly.neg has its own compiled Array.map implementation while sub uses a separate padded binary loop, so these checks cover the intended executable owner surface and their agreement is non-vacuous.

Verification:

• lake build HexPoly.Conformance
• lake build
• git diff --check

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