fix(xim): preserve namespace index identity (0.4.69)

[Sunrisepeak]

Closes #381.

Summary

• upgrade mcpplibs.xpkg to the published 0.0.46 identity model
• preserve every (effective namespace, package.name) candidate in each repo
• resolve explicit namespace:name exactly and report deterministic ambiguity for bare names
• keep bare dependency behavior unchanged: it does not inherit the declaring package namespace
• reject duplicate effective identities and report both descriptor paths
• migrate index cache to v2 with namespace/default-namespace validation
• bump xlings to 0.4.69 and add the reviewed design/implementation documents

Verification

• mcpp build --no-cache --print-fingerprint (GCC 16.1.0, public mcpp-index)
• mcpp test — 11 test programs passed, 0 failed; test_main 234 tests
• bash tests/e2e/index_same_name_namespace_test.sh
• index cache, sub-index search/install, refresh-on-missing, multi-repo interface regressions
• SKIP_NETWORK_VERIFY=1 tools/linux_release.sh — GCC 15.1.0 musl, fully static 0.4.69 archive
• bash tests/e2e/run_all.sh build/xlings-0.4.69-linux-x86_64.tar.gz — 33 passed; the sole local failure is the pre-existing project-context assertion seeing host Node v24.15.0
• bash tests/e2e/custom_index_artifact_test.sh build/xlings-0.4.69-linux-x86_64/bin/xlings

Upstream dependency: openxlings/libxpkg#29 (0.0.46).