Upgrade to DataFusion branch-56 - #124

Summary

Upgrades from DataFusion 52 to the (not yet released) branch-56, plus arrow 60.1, object_store 0.14, and MSRV 1.95.

Note: 56.0.0 is not on crates.io yet (latest is 55.2.0), so the datafusion* dependencies temporarily point at git = "https://github.com/apache/datafusion.git", branch = "branch-56". This blocks cargo publish; they should be switched back to "56" once 56.0.0 is released.

API adaptations

• Drop as_any from TableProvider / ExecutionPlan / PhysicalExpr / ScalarUDFImpl impls; downcast via trait upcasting (plan.downcast_ref::<T>()). UserDefinedLogicalNode still has as_any, so the OneOf downcasts are unchanged.
• ExecutionPlan: properties() returns &Arc<PlanProperties>; implement the now-required apply_expressions; implement replace_children and delegate the deprecated with_new_children to it; replace deprecated partition_statistics with statistics_from_inputs + child_stats_requests (OneOfExec only requests stats for the best candidate).
• OneOfExec declares LimitBehavior::CopyToChildren instead of the deprecated supports_limit_pushdown (the same behavior the old default mapped to).
• ExtensionPlanner::plan_extension / PhysicalPlanner take &dyn Session and a PhysicalPlanningContext; refresh_candidate_readiness now takes &dyn Session.
• TableProvider::scan projection is Option<&[usize]>.
• Replace deprecated TableFunctionImpl::call → call_with_args, LogicalPlanBuilder::scan_with_filters / TableScan::try_new → TableScanBuilder; OptimizerRule::supports_rewrite was removed.
• TableReference / ResolvedTableReference are no longer re-exported from the datafusion_sql root; import from datafusion_common.
• ListingOptions no longer has collect_stat / target_partitions (DF 55); object_store::head now needs ObjectStoreExt.

Regression check

No branch-56-specific regressions found. The only test expectation changes come from behavior changes that landed in DF 54 / 55:

• struct unnest output columns no longer carry the __unnest_placeholder(...) prefix (DF 54, feat: remove __unnest_placeholder from struct unnest projection apache/datafusion#21725) — the "window & unnest" test SQL referenced the old names and now uses date.year etc.
• concat_ws coerces its separator literal to Utf8View (DF 55, feat: support mixed binary and string types for concat UDFs apache/datafusion#22244) — explain text only, results unchanged.

DF 56 rewrote LimitPushdown on top of the new limit_behavior API, so I verified limit pushdown through OneOfExec explicitly: the fetch is still copied into every candidate with no extra limit node, matching DF 52. That check is kept as a new test (tests_limit_pushdown).