Expand Thorium documentation with concrete examples

[edlitmus]

What does this PR do?

Expands the Thorium documentation with concrete cookbook-style examples and adds focused unit tests that exercise the documented patterns.

What issues does this PR fix or reference?

Fixes N/A

Previous Behavior

Thorium's topic documentation and generated reference pages documented only a small subset of the available patterns, with minimal end-to-end examples.

New Behavior

The Thorium docs now include concrete examples for:

• reg.set with check.contains
• threshold gating with reg.list and check.len_gte
• rolling averages with calc.mean
• cooldowns with timer.hold
• wrapper selection across local.cmd, runner.cmd, and wheel.cmd
• register persistence with file.save
• health automation with status.reg and key.timeout

The branch also adds unit coverage for those example flows.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog
  Added changelog/68857.changed.md to record the Thorium documentation and example test coverage expansion.
• Tests written/updated

Commits signed with GPG?

No

Testing

• PYTHONPYCACHEPREFIX=/tmp/codex-pycache python3 -m py_compile salt/thorium/*.py
• git diff --check
• PYTHONPATH=/tmp/salt-test-deps python3 -m pytest -q tests/pytests/unit/thorium/test_examples.py
• PYTHONPATH=/tmp/salt-nox python3 -m nox -e "test-3(coverage=False)" -- tests/pytests/unit/thorium/test_examples.py
• PRE_COMMIT_HOME=/tmp/pre-commit-cache pre-commit run --files ...
  Generic hooks passed and black reformatted the new test file. The remaining Salt-specific hook failures were local environment issues from homebrew pre-commit running under Python 3.14, while this repo only carries requirements/static/ci tool constraints through Python 3.13.

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[dwoz]

This looks great, it just needs a changelog.