Element inspector (M5) is not started: no per-field value / quantization step / ∂r/∂field view, though DataTerm already computes the inputs #33

Plan

Element inspector | ImGuiWidgets.PropertyGrid, one row per OMM field, each showing the value, its quantization step, and the sensitivity ∂r/∂field in metres per ulp at the current horizon. This is the panel that makes Δ_data tangible.

The inspector is also part of the M5 row (L468).

What exists today

• Osculator.Core/Elements/ElementFieldQuantization.cs gives the written step for each field.
• Osculator.Core/Residuals/DataTerm.cs Measure perturbs each field by half a step and returns each field's position effect at a horizon. That is the sensitivity the inspector needs, in km per half-step.
• The app has no panel for element sets.

What's missing

• A panel that lists each field of the selected ElementSet. For each field it shows the value, the step, and metres per step at a horizon the user chooses. It also shows the quadrature total.
• A statement of which representation's steps apply (TLE or OMM). Until ElementSet doesn't record whether it came from TLE or OMM, so Δ_data can't use the right quantization step #27 lands, label them as TLE-text steps.

Acceptance criteria

• The inspector shows every field DataTerm perturbs, in the order Measure returns them.
• It updates when the horizon changes, computing off the UI thread.
• Its numbers match DataTerm.Measure for the same inputs.
• Any new view-model code has a non-UI unit test.
• MeanMotionDot and MeanMotionDdot appear with their measured contribution of exactly zero, as CLAUDE.md describes.

Dependencies

#27 (record whether the source was TLE or OMM, so the right steps show) and #23 (epoch field missing from DataTerm) should land first, or the inspector should reflect them.