Change numeric comparison from double-based to decimal-based

[GuntherRademacher]

This PR aligns numeric comparison semantics with recent specification changes (qt4cg/qtspecs#2218). Mixed-type numeric comparisons now convert both operands to decimals (except for NaN/INF rules), and untyped atomic values are cast to the primitive numeric base type with a fallback to xs:double when required.

All numeric equal and compare implementations were updated accordingly, and the casting logic centralized.

The following invariant holds for all numeric types:

item1.equal(item2, coll, ii) == (item1.compare(item2, coll, false, ii) == 0)

In principle, equal could be removed or implemented in terms of compare. However, its direct implementation remains slightly simpler and is therefore retained.

Tests have been adapted to the changed semantics.

[ChristianGruen]

Pushed (5b3dc01, cb86407).