Fix independent root dependency selection

[Sunrisepeak]

Summary

• Bump mcpp to 0.0.45 for the independent-root dependency fix release.
• Add independent root fallback for bare dependency selectors, so imgui = "0.0.1" tries mcpplibs/imgui first and then root imgui.
• Validate matched xpkg descriptors against package.name / package.namespace before selecting a candidate, preventing an independent imgui.lua from being consumed as mcpplibs.imgui.
• Preserve independent root package identity in mcpp.lock while still recording the default index source.
• Document the imgui-driven mcpp dependency fixes in .agents/docs.

Validation

• mcpp test -> 18 passed; 0 failed
• tests/e2e/63_bare_dependency_peer_root_priority.sh -> OK
• tests/e2e/run_all.sh -> 67 passed, 0 failed, 0 skipped
• tests/e2e/62_dotted_dependency_selector_priority.sh -> OK
• mcpp-index imgui smoke with rebuilt mcpp -> Dear ImGui 1.92.8 module package ok