Add XS fallback mechanism and DateTime Java implementation

[fglock]

Summary

This PR enables CPAN modules with XS code to work on PerlOnJava through automatic fallback mechanisms.

Core Changes

1. XSLoader compatibility - Error message now matches /loadable object/ pattern that modules expect for PP fallback
2. MakeMaker handling - Always installs .pm files for XS modules (with warning); runtime decides fallback
3. Version checking - XSLoader warns (but continues) if Java XS version differs from requested version

DateTime Java Implementation

Full Java XS implementation using java.time APIs:

• JulianFields.RATA_DIE for Rata Die calculations
• Year.isLeap() for leap year checking
• Custom leap seconds table (Java uses smoothed leap seconds)
• All 10 XS functions implemented

Documentation

• New: docs/reference/xs-compatibility.md - Central reference for XS module compatibility
• Updated: docs/guides/using-cpan-modules.md - Added jcpan, XS fallback mechanism
• Updated: Cross-references between all related docs

Files Changed

Java:

• XSLoader.java - Error message + version checking
• DateTime.java - New Java XS implementation

Perl:

• MakeMaker.pm - Simplified XS module handling

Docs:

• docs/reference/xs-compatibility.md (new)
• docs/guides/using-cpan-modules.md
• docs/guides/module-porting.md
• docs/reference/feature-matrix.md
• docs/README.md

How It Works

XSLoader::load('Module::Name', '1.00')
  ├─→ Java class found?
  │     ├─→ Version check (warn if mismatch)
  │     └─→ Use Java implementation (fast path)
  └─→ Not found?
        └─→ Error matches 'loadable object'
              └─→ Module catches error → loads PP fallback

Test Plan

• XSLoader error matches fallback pattern
• Version mismatch produces warning but continues
• Modules without XS_VERSION work silently
• DateTime Java XS loads and works
• All unit tests pass (make)

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