Parent: #3401
Related: #929

Summary

Native LanceDB index creation should accept Lance nested field paths and pass the canonical full path to Lance, instead of resolving columns with a top-level Arrow lookup or falling back to a leaf field name.

Scope

• Update the native Rust create-index path so metadata.user_id, payload.text, and image.embedding resolve through Lance field path semantics.
• Preserve escaped paths such as `a.b` and `meta-data`.`user-id`.
• Ensure scalar, vector, and FTS index creation share the same field-path contract.
• Keep validation errors explicit when the field path is invalid or ambiguous.

Acceptance criteria

• create_scalar_index("metadata.user_id") creates an index on the nested leaf.
• create_index(... vector_column_name="image.embedding" ...) creates a vector index on the nested leaf.
• FTS index creation can target payload.text once the Python wrapper guard is removed.
• Top-level literal-dot fields and nested paths are distinguishable.