Feature: Step() marker for numeric controls without rounding scientific inputs #61

Goal

Introduce Step(value) as an optional numeric editing hint for spin boxes and bounded sliders. This matters for the very small values and heterogeneous physical units in VLab4Mic.

Existing Min, Max, Unit and Widget("slider") remain unchanged. Host-specific implementations can map to magicgui numeric step, SciJava numeric widget step-size, or the corresponding QuPath control. Unsupported hosts retain plain editable numeric fields.

Acceptance criteria

• Declare with Annotated[float, Min(0), Max(1), Step(0.001), Unit("um")].
• Positive finite step only, and only for numeric parameters; misuse raises DeclarationError.
• Step influences increment controls without quantizing or rounding typed values, particularly calibration and microscopy parameters.
• No imposed implicit minimum/maximum or loss of full supported precision.
• Round-trip input tests on all three hosts, CLI, Notebook; add check output, example and manifest documentation.

Priority: Medium; small additive feature.