Add pseudo-orbit BPM reading mode

[thellert]

Summary

• Adds BPMSystem.capture_pseudo_orbit() — injects, tracks N turns, averages per-BPM across turns via nanmean. Returns (n_bpms,) arrays matching orbit interface shape.
• Adds pySCPseudoOrbitInterface that plugs directly into orbit_correction() using the orbit response matrix.
• Extends Tuning.correct_orbit() with pseudo=True, n_turns=N — one-flag switch from closed-orbit to pseudo-orbit correction.
• Fixes pre-existing np.concat → np.concatenate bug in fit_dispersive_orbit().
• Removes the commented-out correct_pseudo_orbit_at_injection prototype.

Motivation

Pseudo-orbit is the natural reading mode between TBT (beam threading) and closed-orbit (stable operation). After stitching achieves multi-turn survival, pseudo-orbit averages out turn-by-turn jitter to give a smoother correction target. This is needed for ALSU accumulator ring commissioning.

@kparasch — I'm open to alternative implementations of this feature, but pseudo-orbit as a first-class reading mode is something we definitely need for ALSU commissioning. It would be nice to have a convenient high-level function for this in pySC. Happy to iterate on the API if you have preferences.

Usage

# Direct reading
x, y = SC.bpm_system.capture_pseudo_orbit(n_turns=20)

# Via interface (for custom correction loops)
from pySC.tuning.pySC_interface import pySCPseudoOrbitInterface
interface = pySCPseudoOrbitInterface(SC=SC, n_turns=20)
x, y = interface.get_orbit()

# One-flag switch on existing correction
SC.tuning.correct_orbit(pseudo=True, n_turns=20, n_reps=3)

Test plan

• 3 new BPM system tests (shape, transmission, single-turn equivalence)
• 1 new tuning smoke test (correct_orbit(pseudo=True))
• All 29 existing tests pass, 0 regressions