refactor(kvs/lance): unify arrow type tree (drop lance::deps workaround)

[AdaWorldAPI]

Summary

Cleanup: drop the lance::deps::arrow_* re-export workaround introduced during the lance-1.0.4 era and unify on plain arrow_array / arrow_schema imports. Resolves the "Arrow type-tree split (workaround in place)" entry in .claude/lance-backend/KNOWN_DIFFERENCES.md.

Why now

Sprint L (#1) pinned lance = "4.0" and arrow-array = arrow-schema = "57". Verified via Cargo.lock + lance-4.0.0 source:

• lance 4.0 internally uses arrow-array/schema = "57.0.0"
• Our pin resolves to 57.3.1 (semver-compatible with 57.0.0)
• Both are the same crate at the same major version → same TypeIds → fully interoperable at the Rust type level

The workaround was needed when:

• Our pin: arrow = "55"
• lance 1.0.4 internally: arrow = "56"
• TypeIds distinct → couldn't pass our types to lance's APIs without conversion → routed through lance::deps::arrow_* re-exports (which gave us v56 types)

After Sprint L's bump, both are v57. The indirection is dead code.

What changes (one file, six sites)

surrealdb/core/src/kvs/lance/mod.rs:

Site	Before	After
Datastore::new (empty RecordBatch builder)	lance::deps::arrow_schema::Schema::new(...) + lance::deps::arrow_array::RecordBatchIterator::new(...)	arrow_schema::Schema::new(...) + arrow_array::RecordBatchIterator::new(...)
Transaction::commit (merge_insert source)	lance::deps::arrow_array::RecordBatchIterator::new(...)	arrow_array::RecordBatchIterator::new(...)
Transaction::get (val downcast)	.downcast_ref::<lance::deps::arrow_array::BinaryArray>()	.downcast_ref::<arrow_array::BinaryArray>()
Transaction::build_write_batch_lance (return + body imports)	lance::deps::arrow_array::* + lance::deps::arrow_schema::*	arrow_array::* + arrow_schema::*
Transaction::scan_impl (2× downcasts for key + val)	same as get	same as get

The doc-comment on build_write_batch_lance is updated to record the unification; the remaining mentions of lance::deps are commentary about the prior workaround for future readers.

Risk

Minimal. lance::deps::arrow_array::T and arrow_array::T are byte-identical at the binary level (same crate, same version). The change is local to import paths; no semantics shift.

Test plan

• CI runs cargo check --features kv-lance --no-default-features — expect 0 errors (target was cleaned locally before this commit; trusting CI for the full rebuild + test sweep per maintainer's call).
• CI runs cargo test --features "kv-lance kv-mem" --no-default-features --lib kvs::lance — expect the existing 50+ kv-lance tests still pass (no semantic change).
• CI default build untouched (no changes to feature gates or feature-default surfaces).

Followups

The remaining items in KNOWN_DIFFERENCES.md after this PR:

• Mixed writes+deletes commit still non-atomic (Dataset::delete separate from MergeInsertBuilder).
• ScanLimit::Bytes proper byte accounting.
• Concurrent-transaction property test.
• Run wider upstream integration suite under SURREAL_TEST_KV=lance.
• Multi-bucket BindSpace sharding (Phase 2 — kv-lance throughput).
• lance-graph Cypher engine as SurrealQL function (Phase 3).
• blasgraph GraphBLAS algebra (Phase 3).
• Benchmarks vs RocksDB / SurrealKv.

The vector-index SIMD work from #2 + #3 (cosine, euclidean, manhattan, chebyshev, pearson via ndarray-hpc) is independent and already merged.

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