Refuse a comparison against NaN rather than ordering it (#947)

[Rafael-SOWNet]

Closes #947.

Real's >, >=, < and <= ordered NaN above every number:

Real one = 1;
Real.NaN > one     // was: true     now: false
Real.NaN >= one    // was: true     now: false
one < Real.NaN     // was: true     now: false

That is EDecimal's total order showing through, and it is not what double does, where every comparison against NaN is false in both directions.

The failure it caused is one-sided, and it is the unsafe side. A guard written as if (value > threshold) treated an undefined value as exceeding the threshold. Of the two possible defaults that is the worse one, and the library's stated position is that answering wrongly is worse than not answering.

CompareTo is deliberately unchanged

This is the part worth reviewing, because it looks like an inconsistency and is not one.

Sorting requires a total order — Array.Sort may loop or throw without one — while an operator requires no such thing. "Where does this sort" and "is this greater" are different questions, and only the second has no answer for a value that is not a number. So CompareTo still puts NaN above every number, and a caller who was relying on the operators to sort should call it instead.

Both halves are pinned by RealComparisonTest, including a test that actually sorts an array containing NaN, because each half reads as a mistake on its own and only makes sense beside the other.

Evidence

• suite 7263 passed, 0 failed, 14 skipped — nothing in the library relied on the old ordering
• the test also pins NaN's comparisons against itself (NaN >= NaN is now false), which is the row most likely to be got wrong from habit
• recorded in BREAKING-CHANGES.md, marked Silent: the call still succeeds and returns the opposite boolean

What it does not do

• Does not touch CompareTo, Equals, or ==.
• Does not touch Entity's comparison operators, which build an inequality node rather than answering a bool — a different thing entirely.
• Does not change how NaN sorts anywhere.

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