Explicit index persistence via save/load commands

[1yefuwang1]

Summary

Replaces the schema-baked index file path with explicit, user-driven persistence. The index path is no longer stored anywhere, so a vectorlite table is fully relocatable.

• CREATE VIRTUAL TABLE is now 2-arg (vector space + index options). The optional 3rd path argument is removed; the 3-arg form is rejected with a helpful error. Breaking change → version bumped 0.2.0 → 0.3.0.
• Persistence is explicit and in-memory-only, driven by INSERT-commands on two hidden columns:

  insert into t(operation, path) values ('save', '/path/index.bin');
  insert into t(operation, path) values ('load', '/path/index.bin');

  These route through xUpdate (which has the VirtualTable* directly), so there's no table-name registry. Works on empty tables, runs exactly once.
• VirtualTable::SaveTo/LoadFrom replace the old auto load/save/delete. LoadFrom builds a new index and swaps it in only on success, validates the file's per-vector data size (label_offset_ - offsetData_) against the table's vector space, and preserves the runtime-only allow_replace_deleted flag (not serialized by hnswlib).
• Removed auto-load on connect, auto-save on disconnect, and file deletion on drop.
• Updated README.md, doc/markdown/api.md, and examples/index_serde.py.

operation, path, and distance are reserved column names. The in-memory index is lost on connection close unless explicitly saved.

Design spec and implementation plan live under docs/superpowers/specs/ and docs/superpowers/plans/.

Test Plan

• C++ unit tests: ctest --test-dir build/dev/vectorlite — 47/47 pass
• Python integration tests: pytest bindings/python/vectorlite_py/test — 13/13 pass, including new tests for save/load round-trip, load-replaces-contents, dimension-mismatch rejection, missing-file rejection, unknown-operation rejection, 3-arg-create rejection, hidden command columns, and allow_replace_deleted preservation across load
• python examples/index_serde.py runs clean (exit 0)