Evaluate compare_ard() keys and columns against both ARDs

[malanbos]

What changes are proposed in this pull request?

• Bug fix in compare_ard() where the keys and columns selections were resolved against x alone and the resulting column positions re-used on y. Each selection is now evaluated against both ARDs, so ARDs holding the same columns in a different order can be compared, and selecting functions may be combined with column names, e.g. keys = c(all_ard_groups("levels"), "stat_name"). (compare_ard() column resolving #606, @malanbos)

.process_keys_arg() and .process_compare_arg() evaluated the tidyselect expression twice. The first evaluation forced the argument, so the second call received the column positions selected from x rather than the expression. Defusing the selection first, with enquo(), means cards_select() receives a quosure on both calls and the selection is made against each ARD in turn. arg_name is now passed through as well, so an unusable selection names the argument it came from.

cards_select() itself is deliberately untouched. process_selectors.R already calls it with defused quosures (expr = x from enquos(...), and expr = lhs_quo), so capturing inside cards_select() would double-capture those.

Tests are added for both symptoms — comparing two ARDs whose columns are in a different order, and keys/columns combining selecting functions with column names. Both fail on main (The keys columns from x and y do not match, and Columns `2` and `3` don't exist) and pass here. The full devtools::test() suite passes, including process_selectors and selectors.

I have left NEWS.md alone — the bullet above is ready to copy in at merge time.

Reference GitHub issue associated with pull request.

Closes #606

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