chore(deps): bump lance 4.0.0 -> 6.0.0, lancedb 0.27.2 -> 0.29.0

Two-major bump for the lance family and one-major bump for lancedb.

Pins updated in surrealdb/core/Cargo.toml:
  - lance       =4.0.0  -> =6.0.0
  - lance-index =4.0.0  -> =6.0.0
  - lancedb     =0.27.2 -> =0.29.0
  - arrow-array  57     -> 58
  - arrow-schema 57     -> 58

Note: lance/lance-index pinned to =6.0.0 (not 6.0.1) because
lancedb 0.29.0 has a strict `=6.0.0` dependency on lance-index.

API surprises (fixed in place; no kv-lance schema changes):
  - DatasetIndexExt moved from lance_index::DatasetIndexExt to
    lance::index::DatasetIndexExt (so Dataset::create_index resolves).
  - lance 6.0 requires arrow 58 (previously 57); arrow-array/schema
    bumped accordingly to unify the dependency graph and resolve
    StreamingWriteSource / RecordBatchReader trait-bound errors.

Files touched:
  - Cargo.toml (workspace comment updated; ndarray patch unchanged
    -- lance-index 6.0 still depends on ndarray 0.16)
  - Cargo.lock (regenerated)
  - surrealdb/core/Cargo.toml (5 dep pins)
  - surrealdb/core/src/kvs/lance/mod.rs (1 import path)

No TODO(lance-bump) markers needed: every call site compiled
in-place after the import fix. `cargo check -p surrealdb-core
--features kv-lance` passes cleanly with no warnings.

https://claude.ai/code/session_01LiUiGeUDLje8KMnxB4FfA3

Author: claude