ci: add nightly dependency canary for JAX/TFP compatibility

[retconrecon]

Adds a scheduled GitHub Actions workflow that tests dynamax against the latest JAX and tfp-nightly every day. If an upcoming upstream release would break dynamax, this opens an issue automatically — typically 1-2 weeks before the breaking version hits stable.

Motivation: #428 went unresolved for 6 months because there was no early warning that TFP 0.25.0 was incompatible with JAX 0.7+. This canary would detect that class of incompatibility the day Google's nightly builds diverge.

What it does:

• Installs latest JAX + tfp-nightly, runs the test suite (Python 3.11 + 3.12)
• On failure: opens a canary-failure issue with the failing matrix jobs and a link to the run
• On recovery: auto-closes the issue

What it doesn't do:

• Does not affect existing CI workflows — runs on schedule only, never on push or PR
• Does not require any secrets beyond the default GITHUB_TOKEN