Tighten missing-dependency install hints so compare-only dependencies point
to cuda-bench[compare] and plotting/dataframe dependencies point to
cuda-bench[plot], instead of defaulting every script to the broader tools
extra.
Also harden nvbench_compare_legacy by reporting missing or skipped state
summaries as UNKNOWN rows instead of silently dropping them, and by converting
missing axis metadata into the existing JSON-structure error path rather than
leaking StopIteration.
Finally, consolidate duplicate finite-number predicates in both compare
scripts so duration formatting and numeric validation share the same helper.
