Add MONTAGE_MUSIC_LOOPS per-movement music loop multiplier

Short adagio movements (Summer Mvt 2 at 107s, Winter Mvt 2 at 132s, etc.)
compress each day to only 3-5 seconds of screen time at 1×.  Looping the
music N times multiplies the per-day target duration proportionally so the
footage is less aggressively sped up and more watchable.

- sky-cam.conf: add MONTAGE_MUSIC_LOOPS and per-movement overrides
  (Summer Mvt2=3×, Winter Mvt2=3×, Autumn Mvt2=2×, Summer Mvt3=2×, Spring Mvt2=2×)
- 4-seasons.sh: multiply target_per_clip by loops
- montage-mvt.sh: compute effective_duration, use -stream_loop for audio,
  fix fade_out_start, -t, and post-build duration check
- make-finals.sh: multiply target by loops

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

4-seasons.sh

@@ -50,10 +50,12 @@ fi
 music_duration=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$music_file")
 echo "Music: $(basename "$music_file")  ($music_duration s)"
 
-# Each daily clip covers music_duration / DAYS_IN_MVT seconds so that
-# concatenating all clips for the movement sums to the exact music duration.
-target_per_clip=$(echo "scale=6; $music_duration / $DAYS_IN_MVT" | bc)
-echo "Target clip duration: $target_per_clip s  ($DAYS_IN_MVT days in movement)"
+# Each daily clip covers (music_duration * loops) / DAYS_IN_MVT seconds.
+# Short movements use loops > 1 so each day gets more screen time.
+loops_var="MONTAGE_MUSIC_LOOPS_${SEASON}_${MVT_NUM}"
+loops="${!loops_var:-${MONTAGE_MUSIC_LOOPS:-1}}"
+target_per_clip=$(echo "scale=6; $music_duration * $loops / $DAYS_IN_MVT" | bc)
+echo "Target clip duration: $target_per_clip s  ($DAYS_IN_MVT days in movement, ${loops}x music)"
 
 # ── Locate yesterday's images ─────────────────────────────────────────────────
 image_dir="$base_dir/$CAM_NAME/$yesterday"