Consider adding DetectorTwoTheta node type to ess.reduce

[SimonHeybrock]

Context

In scipp/essdiffraction#249 we found that assemble_output_calibration in ess.powder had a false dependency on CorrectedDetector when it only needed geometric quantities (Ltotal and two_theta).

ess.reduce already provides DetectorLtotal and MonitorLtotal as sciline node types computed from EmptyDetector + beamline geometry. However, there is no equivalent for two_theta.

As a fix, we introduced DetectorTwoTheta in ess.powder (scipp/essdiffraction#250 — PR pending). This mirrors the pattern of DetectorLtotal:

class DetectorTwoTheta(sciline.Scope[RunType, sc.Variable], sc.Variable):
    """Scattering angle (two-theta) for each detector pixel."""

with a provider that computes it from EmptyDetector + source/sample positions + gravity using the scippneutron beamline coordinate transformation graph.

Question

Should DetectorTwoTheta live in ess.reduce alongside DetectorLtotal? The argument for:

• Symmetric with DetectorLtotal — both are purely geometric, per-pixel quantities derived from the same inputs
• Other instrument packages (not just powder) may need two_theta as a standalone node

The argument against:

• two_theta is only meaningful for scattering instruments, while Ltotal is universally needed (e.g., for time-of-flight computation)
• May be premature to generalize before more instruments need it

For now it lives in ess.powder. Moving it later would be straightforward.