fix(steam): select the declared milk-temperature sensor

[MaxRink]

Summary

Fix SteamSequencer sensor selection so a declared temperature-capable source is selected by its channel contract and remains identity-fenced through lifecycle changes.

• Prefer the declared DecentEspresso/Bengle Milk Probe temperature contract over unrelated object sensors.
• Reselect during an open steam record when the selected source is removed or replaced.
• Preserve the selected probe across registration order, attach/reconnect, unrelated payloads, and E64-like object sensors.

Linked Issue

Fixes #831

Verification

• test/controllers/steam_sequencer_test.dart: 17 passed.
• test/integration/steam_sequencer_integration_test.dart: 1 passed.
• flutter analyze: no issues.
• Parent Linux Flutter suite: 4,158 successful, 1 skipped, 0 failed.
• No hardware claim is made.

Impact

Steam milk-temperature snapshots no longer depend on registry order or an unsuitable sensor's payload shape. This is an isolated sensor-selection bug fix and is a prerequisite for coexistence with the read-only E64 sensor plugin.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.

[tadelv]

Two repository hard gates need to be addressed before this is ready:

1. The Contributor Responsibility checkbox is still unchecked. AGENTS.md explicitly makes the completed PR template a pre-PR hard gate, and this checkbox is the template's responsibility/provenance attestation. Please check it once you've reviewed the submitted changes.

2. The Verification section only states pass counts/results. AGENTS.md requires evidence before assertions (show test output, not just that tests passed). Please add the relevant command output or otherwise provide concrete verification evidence; a successful CI run can serve that purpose once the fork workflow is allowed to run.

I did not find a merge-blocking issue in the sensor-selection implementation itself: selecting by the declared numeric temperature channel, preferring the exact Bengle Milk Probe contract, and reselecting on registry changes are consistent with #831.

[tadelv]

Rechecked after the force-push/rebase. I don't see a remaining code-level blocker in the current diff, and the Contributor Responsibility checkbox is now fixed.

The current CI failure is stale rather than a code failure: the workflow run was created before the checkbox was updated, and re-running the failed contribution-policy job replays that original PR event payload, so it still sees the checkbox as unchecked and skips the actual analyze/test/build jobs.

Please trigger a fresh pull_request workflow event (for example via another synchronize/push, or close/reopen the PR) so CI evaluates the current PR body and runs the real checks. Once that is green, I have no further implementation changes to request.