Convert the published PhAIL dataset to `joint_signals` and drop the singular bridge

[vertix]

The 3D viewer locates joint signals through joint_signals (plural). #584 migrated every producer to it, but the published PhAIL v1.0 dataset has the singular joint_signal baked into its static.json, so dataset_utils._collect_signal_groups still folds the singular key in:

# TODO(#587): drop once the published PhAIL dataset carries the plural key.

That bridge is the only thing keeping the retired name alive in the reader. It goes away once the data carries the plural key.

Why the dataset has the singular key

phail_teleop_release was migrated with REAL_ROBOT_TRANSFORM applied, and ROBOT_SIGNAL_POINTERS wrote joint_signal at the time (positronic/cfg/ds/internal.py). migrate_dataset materializes static into static.json (positronic/dataset/utilities/migrate_remote.py:28), so the value is on S3, not computed at read time.

The public viewer config phail_with_started now applies ROBOT_SIGNAL_POINTERS (#584), so the PhAIL viewer itself is already correct — the bridge covers reading the released data any other way, e.g. a bare positronic-server --dataset.path=s3://….

Scope

Only the published PhAIL dataset. Internal raw datasets are read through configs that apply ROBOT_SIGNAL_POINTERS, and recordings made after #584 write the plural key directly.

Steps

1. Re-publish PhAIL v1.0's static with joint_signals. migrate_dataset with the current transform rewrites the dataset; EditedDataset.set_static appends the key per episode to an edits.jsonl beside the recordings instead, which the default load_dataset/load_all_datasets path already applies — no episode rewritten. Pick whichever fits how the release is regenerated.
2. Delete the singular branch from _collect_signal_groups and the TODO that names this issue.
3. Confirm in the viewer that a released episode still shows its arm model and joint1…joint7 legends.