feat(plustek): add Multi-Pass ME scanning

[TobbyTravel]

Highlights

• New: Multi-Pass mode. Merge 2-9 exposures per frame for lower shadow
  noise, at a roughly proportional cost in scan time. pyopticfilm's own
  benchmark on an 8100 V2 (5 exposures vs. today's 2, same frame): -26%
  relative chroma noise, -28% relative luma noise, visibly smoother in flat
  regions.
• Clearer control. The old "Multi-exposure" checkbox — which secretly
  only ever meant one specific behavior — is now a single dropdown: Off /
  Dynamic (2 exposures) / Fixed (2 exposures) / Multi-Pass (2-9). Choosing
  a mode is now an explicit decision instead of a hidden default.
• Tidier scan setup. Frame selection, scan-window/Prescan, and Clear now
  live together as one full-width block right above the Scan button — the
  last thing you touch before scanning, grouped where you touch it. Crop
  info, when a crop is set, shows next to Frame info instead of its own row.
• No new risk surface. Every mode is a name for a fully validated
  pipeline path — there's no raw exposure field, no way to type in an unsafe
  value. The debug-only raw exposure overrides pyopticfilm keeps for its own
  ScanLab tooling are never called from NegPy.
• Fully backward compatible. Off stays the default; Dynamic at 2
  exposures is byte-identical to today's only multi-exposure behavior. No
  existing scan or saved setting changes.

Technical changes in NegPy

• ScanParams/ScannerSettings gain a MultiExposureMode enum replacing
  multi_exposure: bool, plus a bracket count meaningful only in Multi-Pass.
  Old settings blobs migrate automatically (multi_exposure: true →
  dynamic, false → off).
• plustek_backend.py translates the mode into pyopticfilm's
  scan(multi_exposure=, me_exposure_mode=, n_brackets=), and generalizes
  the previously 2-pass-only scan progress model to N passes.
• ScanSidebar: mode combo + bracket slider replace the checkbox; the
  Framing section (frame spec, scan window, Prescan) moves to a full-width
  block above the Scan button; crop info relocates next to Frame info.

Why this is a PR draft

This depends on pyopticfilm API surface (n_brackets, me_exposure_mode
widened to str | None, Model.me_default_exposure_mode) that only exists
on pyopticfilm's feat/me-n-brackets branch (#47), not in any released
version yet. This will stay a draft until that pyopticfilm work is merged
and released, since it can't be installed as a normal dependency or fully
verified against a real release until then.

[TobbyTravel]

Closing in favor of #1064.

This branch was built against an earlier pyopticfilm design (N different exposures merged, like HDR bracketing) that never shipped. pyopticfilm instead shipped PR #65 with a different mechanism — repeating a single exposure n_passes times and stacking the aligned repeats for an SNR gain, orthogonal to the existing ME short+long fusion rather than an alternative to it (n_brackets isn't part of that API). #1064 reworks the UI/backend wiring for that actual API.