style/performance fixes - #7086

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Merged, thank you. Both halves are changes I want, and the second one is a convention I wrote into CONTRIBUTING.md a few days ago that nothing had gone back and applied.

What I checked before merging, since a sweep across nineteen files is the kind where a single bad substitution hides easily:

• The 39 block changes are all block.call(...), none of them &.call, so nothing that was guarding against a missing block lost its guard. map, select, each_with_index, collect! and friends still return an Enumerator when called without one, and max/min still fall back to <=>.
• The 28 type checks all kept the class on the left and the value on the right. Array === row, not the reverse.
• Twenty rows against CRuby 4.0.6, block-present and block-absent, all agreeing. Full suite green, ASan and UBSan clean, no new warning.

One request for next time, and it is about the shape rather than the content: please split a sweep like this into one pull request per kind of change. block.call to yield and is_a? to Module#=== are independent, and each would stand alone.

The reason is collision surface rather than review effort. This landed alongside a run of unrelated work in mrblib and mrbgems/*/mrblib, and a nineteen-file diff has to be rebased whole every time any of it moves. Two smaller ones would each have moved through on their own. It also lets one be reverted without the other if something turns up later.

Nothing to change here, and I would rather have the sweep than not have it. Thank you for doing the unglamorous half of a convention.