Add configurable scalar index on id column

[beinan]

Summary

• Add IdIndexType enum (None/BTree/ZoneMap) for configurable scalar index on the id column
• BTree indices are maintained by MemWAL during writes; ZoneMap indices are excluded from MemWAL maintenance (unsupported by MemWAL) and rebuilt after compaction
• Index is created eagerly on store open and ensured after each compaction cycle
• Exposed via Python API as id_index_type parameter ("btree", "zonemap", or None)

Test plan

• Rust tests: btree index creation on open, persistence after compaction
• Rust tests: zonemap index creation on open, persistence after compaction
• Rust tests: no index created by default (IdIndexType::None)
• Rust tests: idempotent ensure_id_index (no version bump on re-check)
• Python tests: btree/zonemap creation, invalid type rejection, data integrity across compaction cycles
• Full existing test suite passes (19/19)

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