feat(api): per-field verification for POST /machine/settings

[ChampionDesigns]

Summary

POST /api/v1/machine/settings answered a bare 202, so a client could not tell whether the
machine took the value it asked for. It now reads every written field back inside the same
serialized device write and answers with a per-field report:

{"results": {"fan": {"requested": 51, "actual": 50, "status": "adjusted"}}}

Base: main. Independent.

Design notes

MachineSettingWriteStatus is applied when the read-back matches within half of one write
quantization step, adjusted when the machine took something else, and unverified when the
read-back itself failed. A read-back timeout is reported as unverified, never mistaken for a
successful write.

MockDe1.setFanThreshhold now clamps to MMRItem.fanThreshold's own min/max, as the firmware
does. Without that the mock could hold a value the machine never could, and a read-back
verification cannot tell applied from adjusted.

That correction exposed a pre-existing defect: de1handler_settings_reset_test expected a fan
threshold of 55, which is above the firmware's max of 50 and was never reachable. The
unclamped mock was hiding it. It now expects 50.

Linked Issue

N/A

Verification

• flutter analyze — clean.
• flutter test — the three settings test files 40 passed, and the full suite 3931 passed /
  1 skipped after the mock clamp, which touches every test that writes a fan threshold.
• dart format — clean on every changed file.
• Verified on hardware. This change ships in the Decaid-Canary build Ben runs on his own
  machine, and has been exercised in normal use rather than only under test.

Impact

• API: POST /api/v1/machine/settings now returns {"results": {...}} in the 202 body where
  it returned a bare ack. assets/api/rest_v1.yml gains De1SettingsWriteReport and
  De1SettingWriteResult; doc/Api.md documents them.
• Compatibility: additive. A client that ignores the body is unaffected; the status code does
  not change.
• User-visible: none directly. A skin can now show whether a setting took.
• Behaviour: MockDe1 is stricter — it clamps where it did not. Tests that relied on the mock
  holding an out-of-range value will now see the clamped value, which is what the machine does.
• Security: none.

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.