Build propagator transfer functions on the device the data is on

[ShantanuKodgirwar]

Fourth in the series building a safety net before performance work. Follows #90, #91, and the
e3PIE fix.

The bug

Four propagators passed params.gpuSwitch into their cached transfer-function builders instead
of asking where the field actually is:

function	line
propagate_twoStepPolychrome	Operators.py:271
propagate_scaledASP	Operators.py:343
propagate_scaledPolychromeASP	Operators.py:415
propagate_polychromeASP	Operators.py:495

gpuSwitch is a global intent flag, not a fact about any particular array. Arrays get moved
by BaseEngine._checkGPU and by the engines themselves, independently of when a transfer
function is first built and cached — so the two can disagree. Then a CPU transfer function meets
a GPU field:

TypeError: Unsupported type <class 'numpy.ndarray'>

All four now use isGpuArray(fields). Worth noting the other two transfer-function builders in
the same file (Operators.py:101,147) already did this — these four were the outliers, not
a new convention.

Verification

Each of the four call sites is covered by a GPU-vs-CPU test that passes a device array while
leaving params.gpuSwitch False. Reverting Operators.py to main fails all four:

FAILED test_propagator_gpu_matches_cpu[scaledasp]
FAILED test_propagator_gpu_matches_cpu[polychromeasp]
FAILED test_propagator_gpu_matches_cpu[scaledpolychromeasp]
FAILED test_propagator_gpu_matches_cpu[twosteppolychrome]

Three are polychrome and need nlambda > 1, so they live in tests/regression/ where the
synthetic dataset can supply a spectral density; the single-wavelength ones are in
tests/Operators/.

Other test coverage added

test_object2detector previously called every propagator and asserted nothing. Now:

• output goldens for all eight propagators
• round-trip identity for the unitary ones (fraunhofer, fresnel, identity)
• linearity, P(ax) == aP(x), for all of them

Suite goes 30 → 59 passing.

Why round-trip identity is not asserted for asp/scaledasp

They are band-limited — forward∘inverse discards out-of-band energy, so it is not the identity.
Measured ||P(x)-x||/||x||: 3.4e-01 for asp, 5.4e-02 for scaledasp.

What survives differs between them, which is why neither is asserted:

	round-trip vs identity	idempotent?
asp	3.4e-01	yes — 3.1e-07, a clean 0/1 cutoff
scaledasp	5.4e-02	no — 1.6e-03, its chirp/rescaling factors are not

Linearity is the property both genuinely share; the numbers themselves are pinned by the
goldens.

Version

0.2.7 → 0.2.8, published automatically on merge.