Split De Morgan's laws into one theorem per law (propositions and quantifiers) #603

The mds rule "a theorem-type div states one result" (CLAUDE.md, "One term per #def- div") is broken by two theorems that each state a pair of laws:

• _subfiles/proof-writing/_thm-de-morgan-logic.qmd (thm-de-morgan-logic), on main;
• _subfiles/proof-writing/_thm-negate-quantifiers.qmd (thm-negate-quantifiers), added by Add domain of discourse, quantifier negation and cycles of conditionals to Proof Writing #597.

The review of #597 at faed525 flagged the second one (#597 (comment)) and noted the first.
#597 already holds 10 files, the per-PR cap, so the split is deferred here and should handle both pairs the same way.

Proposed fix, for each pair:

• keep the existing id on the first law, so links survive;
• give the second law its own thm- div, proof and exm- example;
• reword or drop the remark that explains why the two laws share a theorem (rem-negate-quantifiers);
• repoint any link that means the second law.

mcs links to thm-de-morgan-logic too, so check its uses when the mcs switch PRs land.

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