Admin Savoirs Faire Coverage

Per grade × chapter × savoir-faire exercise counts (admin diag).

Strategy: load programmes (grade tree + chapter list) and savoirs_faire once, then run a single aggregation over public_exercises grouping by (grade_display, chapter_display, sf_id, difficulty). The aggregation excludes Legacy + remediation rows so the counts match what teachers actually browse. We $unwind savoirs_faire_ids with preserveNullAndEmptyArrays so untagged rows still feed the chapter-level "untagged_exercise_count" tally.