feat(vamana): Add relabelVector, a data-preserving external-ID rename

[nonirosenfeldredis]

Summary

MutableVamanaIndex::delete_entries + add_points was the only existing way to change a
vector's external label -- and that path re-reads and re-inserts the vector's data, and
re-links its graph adjacency, even when the caller only wants a new label for data that
hasn't changed. That's exactly the situation a downstream index-update path faces after a
document update turns out not to have touched the vector: nothing about the stored vector or
its graph position needs to change, only the external ID it's known by.

• IDTranslator::remap_external_id(from, to): the missing symmetric counterpart to the
  existing remap_internal_id (used internally by compact()) -- a pure O(1) hash-map key
  swap on the external ID, checked by default the same way insert/delete_* are.
• MutableVamanaIndex::relabelVector(old_id, new_id): exposes this at the index level,
  alongside the existing translate_external_id/has_id/get_datum methods it's meant to be
  used with. Touches neither the dataset nor the graph.

Testing

• New "Rename External" section (plus two error-path sections) in
  tests/svs/core/translation.cpp.
• New "MutableVamana Index Relabel" test case in tests/svs/index/vamana/dynamic_index.cpp,
  verifying the internal id, stored data, and every other id are unaffected by the rename.
• Verified both error-path sections actually exercise the check (not just happen to pass) by
  temporarily disabling it and confirming all four throw-assertions fail as expected, then
  restoring it.