Break up ensure_distribution_with_stats and enforce_distribution_relationships in EnforceDistribution #26299

Is your feature request related to a problem or challenge?

EnforceDistribution concentrates most of its decision-making in two long functions in enforce_distribution.rs, which has had 21 commits in the last 12 months:

• ensure_distribution_with_stats (L1359-L1800): cognitive complexity 82, cyclomatic 52, nesting depth 6. A large closure passed to map() (roughly L1445-L1595) computes the subset-satisfaction flags, file-partition preservation, round-robin decisions and the Range/Hash repartition choice inline. Range-partition scaling with its fallback is nested five levels deep inside the HashPartitioned arm. A second closure (about L1610-L1700) mixes ordering-requirement handling with distribution-dependent branching.
• enforce_distribution_relationships (L1120-L1323): cognitive complexity 65. It mixes reference-child selection with the child rewrite that unwraps or wraps a RepartitionExec using mem::replace/pop.

The boolean flags (allow_subset_satisfy_partitioning, preserve_satisfying_file_partitioning, needs_repartition, ...) interact subtly and cannot be tested on their own. Interactions in this rule have caused real bugs (for example the case referenced via #18341 and the partitioned-join panic described in a code comment).

Describe the solution you'd like

• Compute the per-child policy flags in a small ChildPolicy struct built by a dedicated function with unit tests.
• Extract the per-child closure into ensure_child_distribution(...), the Range scaling block into something like scaled_or_fallback_partitioning(...), and the ordering closure into enforce_child_ordering(...).
• In enforce_distribution_relationships, extract find_reference_child, partitioning_already_satisfied and insert_repartition so the main loop only drives the fixpoint.

Each step can be a behaviour-preserving PR relying on the existing enforce_distribution tests and plan snapshots.

Describe alternatives you've considered

None. The logic is correct today; the goal is to make the next change to it safe.

Additional context

Line references are pinned to commit 0d79252.