Every tense in conjugation_paradigm/1 now carries a :level key (the
CEFR/JLPT/HSK key at which the tense/mood is typically taught) — a
property of the tense in the curriculum, independent of any verb. All
16 French tenses are levelled A1→C2.

Adds tense_level/3 as a convenience reader over the paradigm tree.
Purely additive; existing keys and consumers are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>