feat(xpkg): support regional source maps

[Sunrisepeak]

Summary

• add xpm.source = { GLOBAL = ..., CN = ... } parsing
• preserve legacy string source and version-level URL/mirror behavior
• normalize root/platform source maps into the existing resolver output
• default to GLOBAL while exposing CN and other regions as fallback candidates
• bump the package version to 0.0.45

Verification

• mcpp test -- --gtest_filter='*SourceMap*'
• mcpp test