ePIE.reconstruct() runs the reconstruction as it should

[ShantanuKodgirwar]

Second in the series that builds a safety net before performance work. Follows #90.

The bug

ePIE.reconstruct() contained a yield, which makes it a generator function. Calling it
builds a generator object and returns immediately, without running anything.

Most callers call it plainly:

engine_ePIE.reconstruct() # example_scripts/exampleReconstruction_pcPIE.py:126

So those runs have been silently producing an unreconstructed object. The tutorials do the
same.

The fix

reconstruct() now drains the loop to completion. The generator behaviour is kept as
reconstruct_stepwise(), for callers that want to interleave their own work between scan
positions.

Also removed: the unconditional CUDA stream

The position body was wrapped in:

withcp.cuda.Stream(non_blocking=True) asstream:

Two problems:

• Without CuPy installed, cp is None, so this raises AttributeError. ePIE could not run
  on CPU at all.
• stream.synchronize() fired after every scan position, forcing a host/device round trip for
  no benefit.

Both are gone.

Test coverage

Five ePIE goldens added: Fraunhofer, ASP, Fresnel, mixed-state (nosm=2, npsm=3), polychrome
(nlambda=3). Suite goes 20 → 30 passing.

Verified the goldens are meaningful by running them against the unfixed engine — all ten fail:

• the CPU goldens mismatch the recorded arrays;
• the GPU-vs-CPU comparison raises AttributeError: 'Reconstruction' object has no attribute 'error', because _prepareReconstruction() lives inside the generator body and never ran.

⚠️ Breaking change

Anyone doing for _ in engine.reconstruct(): must switch to reconstruct_stepwise().

Updated in this PR:

file	note
tests/Engines/test_propagator.py	genuinely affected
docs/fpm/overview.md	already broken on main
docs/advanced/multislice.md	already broken on main

The two doc snippets iterate the FPM and e3PIE engines, neither of which was ever a generator
— for loop, posLoop in engine.reconstruct() would have raised TypeError there today. ePIE was
the only generator in the codebase.

Version

0.2.5 → 0.2.6. Not published on its own; grouped with the next two fixes into one release.