expose the firmware's live LED registers, so a colour can be shown without storing it

[ChampionDesigns]

Summary

The Bengle firmware separates showing a colour from deciding one, and this app wired only
half of it.

FrontLEDColor / RearLEDColor            0x00803890/94   F_LEDStripColor
    sets the model and calls sendLEDColors() — the strip lights at once, and
    applyLEDsForGivenState recomputes these from the stored pair at the next
    sleep or wake transition. Showing a colour, not deciding one.

FrontLED{Awake,Sleep} / RearLED{Awake,Sleep}  0x00803898..A4  F_LEDStoreColor
    stores the colour, and applies it ONLY when the machine is already in the
    state that colour belongs to.

Only the stored four were declared. So every colour a client sent was a decision, and an asleep
colour could not be shown at all: written while the machine is awake it is kept and never lit,
which reads as the machine ignoring you. That is the complaint this fixes.

Base: main. Independent.

Design notes

• frontLedColor / rearLedColor join BengleMmr at the firmware's own addresses.
• previewLedStrip({front, back}) writes only the live pair. The stored palette is untouched,
  so ledStripState does not move — a preview cannot become a decision by accident.
• POST /api/v1/machine/ledStrip/preview exposes it. Body {"frontStrip": "<12 hex>", "backStrip": "<12 hex>"}, either optional, in the same colour spelling GET /machine/ledStrip
  uses.

Linked Issue

N/A

Verification

• flutter analyze — clean.
• flutter test — full suite 3894 passed against current main, including
  bengle_led_preview_mmr_test.
• 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: adds POST /api/v1/machine/ledStrip/preview, documented in assets/api/rest_v1.yml.
  Additive.
• Compatibility: the stored-palette path is unchanged. A client that never previews sees no
  difference.
• User-visible: a colour picker can finally show a colour without committing it, including an
  asleep colour on an awake machine.
• 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.