deps(core): align kv-lance pins with lance-graph surrealdb#423 (lance 4→6, lancedb 0.27→0.29, arrow 57→58)

Catches the AdaWorldAPI/surrealdb fork up to lance-graph main (post surrealdb#423):
- lance        4.0.0  → 6.0.0  (exact-equals)
- lance-index  4.0.0  → 6.0.0  (lock-step)
- lancedb      0.27.2 → 0.29.0 (exact-equals; transitively pins lance =6.0.0)
- arrow-array  57     → 58     (lock-step with datafusion 53 in lance-graph)
- arrow-schema 57     → 58

All bumps under the optional `kv-lance` feature (no change to default build).
Cargo.toml-only; no .rs touched.

Note: the user-authorised follow-on patch to lance =6.0.1 is currently BLOCKED
by lancedb 0.29.0's transitive `lance = "=6.0.0"` requirement (lancedb 0.29.0
is the latest 0.29.x on crates.io). See lance-graph root Cargo.toml RESOLVED(A)
for the full reasoning. Re-bump when lancedb 0.29.1+ ships.

https://claude.ai/code/session_01FMooFcE7hgRWWvknNr2N4i

Author: claude

surrealdb/core/Cargo.toml

@@ -107,11 +107,11 @@ surrealdb-types = { workspace = true }
 vart.workspace = true
 
 # External KV stores
-lance = { version = "=4.0.0", optional = true }
-lance-index = { version = "=4.0.0", optional = true }
-lancedb = { version = "=0.27.2", optional = true }
-arrow-array = { version = "57", optional = true }
-arrow-schema = { version = "57", optional = true }
+lance = { version = "=6.0.0", optional = true }
+lance-index = { version = "=6.0.0", optional = true }
+lancedb = { version = "=0.29.0", optional = true }
+arrow-array = { version = "58", optional = true }
+arrow-schema = { version = "58", optional = true }
 rocksdb = { workspace = true, optional = true }
 tikv = { workspace = true, optional = true }