Require camera name explicitly across all seasons scripts

No script defaults to SUNRISE_CAM (east) for the seasons pipeline.
Every direction is treated equally — camera name is a required arg,
not an optional override.

- 4-seasons.sh: $1 required, exits with usage if missing
- montage-mvt.sh: arg order changed to <cam> [date]; $1 required
- year-end-join.sh: $2 (cam) required
- verify-mvt.sh: $4 (cam) required
- migrate-seasons.sh: example commands updated to new arg order
- README.md: montage-mvt.sh examples updated

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

README.md

@@ -227,8 +227,8 @@ Attribution data for every file is written to `sunrise-sounds/manifest.json`.
 
 **Rebuild a movement montage** (e.g. to retry audio after a failure):
 ```bash
-./montage-mvt.sh                        # uses today's movement
-./montage-mvt.sh 2025-06-15 east        # specific date + camera
+./montage-mvt.sh east                   # uses today's date
+./montage-mvt.sh east 2025-06-15        # specific date
 ```
 
 **Rebuild the year-end video**: