demo: use ControllerVector for temperature ramp sub-controllers

[coretl]

Closes #390

Evolves src/fastcs/demo/controllers.py's composition example onto the documented ControllerVector pattern (see docs/explanations/controllers.md) instead of a manual list + add_sub_controller loop, and adds unit tests exercising cancel_all and the voltage-distributing @scan against a mocked IPConnection.

This is the current-API baseline described in the issue (Float(...), io_ref=...); the getter/setter cleanup lands separately when #392 lands.

Instructions to reviewer on how to test:

1. uv run pytest tests/demo/test_controllers.py -v
2. Confirm the temperature controller demo still runs against the sim (python -m fastcs.demo run src/fastcs/demo/fastcs.yaml)

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes

Notes

• Verified locally with uv run --locked tox -e pre-commit,type-checking,tests. pre-commit and type-checking pass. In the sandbox this ran in, the tests env's plain pytest collection also picks up docs/conf.py (needs outbound network) and tests/benchmarking (needs a PVA-capable socket family), both of which fail to even collect for reasons unrelated to this change. Excluding those two paths, pytest src tests --ignore=tests/benchmarking passes 310/320 with only PVA (p4p) IOC tests failing on RuntimeError: Address family not supported by protocol — the same known sandbox limitation. Real CI covers these.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (3da025f) to head (1932e5e).
⚠️ Report is 1 commits behind head on refactor.

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor     #409   +/-   ##
=========================================
  Coverage     91.25%   91.25%           
=========================================
  Files            72       72           
  Lines          2892     2892           
=========================================
  Hits           2639     2639           
  Misses          253      253           

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