Negative sets as data (#341)

* Add article: Negative Sets as Data

Cofinite sets with one sentinel key — all set operations reduce
to three map primitives (merge, keep, remove). No special types,
no wrapper objects, no case explosion.

Includes working Clojure implementation, verification of all four
polarity combinations, and proofs of De Morgan's laws, identity
elements, and self-complement properties.

* rewording and minor tweaks

* Clarifications

- Use map-* operations in table
- specify contains? in Membership

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Co-authored-by: Gorm <gormbot@jclaggett.dev>

Author: jclaggett