Implement dependency usage requirements

[Sunrisepeak]

Summary

• add dependency visibility parsing (public/private/interface) with legacy public default
• add resolved include usage requirements on PackageRoot and make scanner/build-plan use them
• stop include propagation from mutating parsed manifests; keep package-owned cflags/cxxflags private
• keep the C++ include_flags assembly fix and stale unmarked xpkg cache recovery
• document the architecture and local verification in .agents/docs

Verification

• mcpp build
• mcpp test (18 passed; 0 failed)
• tests/e2e/run_all.sh (65 passed; 0 failed; 0 skipped)

Release note

Per current plan, do not trigger the 0.0.43 release from this PR. Version bump, release trigger, xim-pkgindex update, and xlings-res rollout are deferred until the next requested scheme lands.