feat: add calibrated scalar maps with Plot2D

[bobleesj]

New public widgets and shared-helper refactors need an issue or maintainer
discussion first. Bug fixes and features that stay inside one existing widget
can open a pull request directly. See
pull request classes.

What problem does it solve?

Adds Plot2D for existing scalar maps with physical axes and a labeled color
scale, without conflating them with spatial Show2D images. Public API:
quantem.widget.Plot2D(data, *, x, y, ...), set_data(data) and
figure(); data are not scientifically transformed.

What should reviewer(s) do?

After the required maintainer discussion, review calibrated row/column behavior
and the default static-preview / opt-in full-state policy. Drive P2D-01 through
P2D-08, including actual notebook-manager payload and save/reopen behavior,
before approving. This proposal has no standalone HTML export.

Verification

• Build, typecheck and 180 frontend tests pass, including eight Plot2D tests.
• Actual installed-wheel suite: 1177 passed, 29 skipped, one unchanged
  PR-hygiene/workflow assertion failed. The 118 Plot2D/shared saved-state tests pass.
• Tutorial executes against the wheel; canonical notebook contains no outputs
  or baked state. Independent review includes TkAgg and calibrated preview checks.
• GitHub signoff failed during frontend build: current GPU source re-exports
  display/backends/webgpu/colormaps, which the shared sync manifest omits.
  CI failure.
• Pending: live browser pixels/gestures/PNG, actual saved-notebook payload and
  preview freshness, latency, and public GPU dependency availability. Tests use
  an explicitly pinned compatible GPU wheel; PyPI cannot resolve required rc6.

---

This PR follows the
scikit-package workflow for
reproducible scientific software.

[github-actions]

• Delete bobleesj/quantem.widget:sep-08-plot2d-review from its remote if you own it, it is not long-lived, and no other PR uses it.
• Remove the dedicated worktree and local sep-08-plot2d-review branch after confirming the worktree is clean.
• Fetch electronmicroscopy/quantem.widget and update local main before starting new work.