Add support for fixed default element namespace

[GuntherRademacher]

This PR adds support for the XQuery 4.0 feature declare fixed default element namespace.

When fixed is declared, the default element/type namespace is no longer affected by xmlns declarations on direct element constructors. To support this, the static context now distinguishes between:

• the default namespace used for expression QName resolution (elemNS)
• the default namespace used for direct element constructors (dirNS)

QName resolution was adjusted accordingly, and deferred QName resolution during parsing was updated to use the correct namespace context. Minor cleanups were included (avoiding entries for already-resolved QNames).

A new NamespaceTest.fixed() test method was added to cover the new behavior. Also these QT4 tests now pass successfully:

• default-namespace-40-01
• default-namespace-40-02
• default-namespace-40-03
• default-namespace-40-04

The second commit in this PR refactors QNmCache and QNmCheck, because their previous naming was somewhat misleading. The classes are replaced by QNmResolver that stores deferred resolution entries in a small record. The refactoring simplifies the control flow, removes redundant checks, and makes the intent of the code - resolving namespace URIs of QNames once the namespace context is known - more explicit.

[ChristianGruen]

Looks all good. And thanks for introducing the QNmResolver class, it is much more intuitive than what we had before.