fix: bind a RIGHT join's merge arguments to the declared sides

[TKaltofen]

What changed

A JoinType.RIGHT link between two feature groups that resolve to different compute frameworks bound its merge arguments to the resolved frameworks instead of the declared sides.

run_link swaps destination and source framework for RIGHT so the join executes where the declared right group runs, but left swap_merge_sides False. JoinStep._merge_data then handed the destination's data, the declared right group, to the merge engine as its left argument, while the link's indices stay bound to the declared feature groups. Left keys a, b, c against right keys a, b returned three rows where a RIGHT join returns two. Where the index column names did not line up it raised a KeyError naming the declared left index instead.

The flag now follows which declared side the destination framework actually holds, read off the parents in the graph, rather than the position of a framework in the trekker key. Setting it for every RIGHT join would be wrong in the other direction: when the key reaching run_link is already reversed the destination is the declared left group, and False is correct there.

Sides are resolved by nearest inheritance distance rather than plain issubclass membership, because membership answers "both sides" in two shapes that then fall through to the old behavior:

• a declared right group that subclasses the declared left group matches both sides itself
• the parents walked at that point are the transitive ancestor closure of the join's children, not the join's two parents, so an unrelated ancestor that subclasses a declared side and runs on the destination framework contaminates the answer

Keeping only the closest nodes per side separates a subclass from the base it derives from and drops a distant sibling in favour of the real parent. The MRO distance helper moved to module scope so the link matcher and the planner share one.

Self links, and two subclasses of one declared side at equal distance on different frameworks, stay undecidable from the declared sides and keep the previous value.

Behavior change

RIGHT joins across compute frameworks now return the join the link declares. Runs that previously raised will now succeed, and runs that returned rows from a join with the sides exchanged will return different rows.

Tests

New end-to-end regression coverage for a plain RIGHT join across frameworks, for a declared right group that subclasses the declared left one, and for a sibling subclass reaching the same child as a second ancestor, all with asymmetric key sets so a wrong binding changes the row set rather than only raising. Plan-level coverage for the reversed-key and inverted-branch orientations that must not swap. The existing planner characterization suites are updated where they pinned the old results.

tox is green: 8978 passed, 170 skipped, 2 xfailed.