feat: Modal recipe (Wave 3 opening, ML)

[zuchka]

Summary

• Opens Wave 3 of the platform recipe program (Wave 2 closed with v0.8.0 / Ray).
• Adds a Tier-1 recipe at docs/recipes/modal.md showing how DING wraps modal run locally, ingesting JSON events streamed back from the remote function. Modal's container architecture forces this local-wrap pattern (the Modal runtime owns the container entrypoint, so ding run -- can't wrap inside the container).
• No Go changes. runctx is unchanged. Recipe-only PR.
• Modal lands in the existing ML category alongside MLflow + Ray. README runner table is NOT modified — Modal isn't auto-detected.

Tier verdict

Tier 1 with Tier-2 candidate flagged: pip install ding-modal Python helper providing a decorator that auto-emits Modal task metadata (parallel to the ding-ray Tier-2 candidate from Wave 2).

Test plan

• Smoke gate (HARD — recipe is NOT shipped until this passes):
  ```bash
  pip install modal
  modal token new # one-time
  mkdir /tmp/modal-smoke && cd /tmp/modal-smoke

  Author app.py + ding.yaml from the recipe; trainer() prints val_loss=12.4 then sys.exit(1)

  export SLACK_WEBHOOK_URL=...
  ding run --config ding.yaml -- modal run app.py::trainer

  Verify:

  1. loss_spike Slack alert (val_loss > 10) mid-execution

  2. training_failed Slack alert with exit 1

  3. modal_task_id label matches the Modal dashboard's task ID

  ```
• mkdocs build --strict (CI runs it; not run locally — mkdocs isn't installed)

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