Review fixes for #166 (backend separation)

[oliviermattelaer]

Fixes for the review of #166, as a PR into backend_seperation_PR so they can be looked at (and accepted or dropped) separately from the split itself. One commit per item, in the order of the review comment; each commit message says what was wrong, how it was reproduced and how the fix was checked, so any of them can be reverted on its own.

Needs fixing (review items 1-3), plus three regressions found while fixing them

Commit	What
a2c5a52	(1) nMF back in scope for initProc: every merged-flavor model (e.g. MSSM u u~ > n1 n1) failed to compile. Now builds and reproduces the merge-base ME.
4f37adb	(new) Color choice goes through the ncolor_flow flows again. The split hard-coded jamp2_sv[ncolor*iParity+icol] += cxabs2(jamp_sv[icol]), dropping the per-process jampflow_lines. For DDM (all-gluon) processes ncolor != ncolor_flow: on g g > g g g (6 DDM jamps, 24 flows) the split selected 12 distinct flows, one of them 568/1024 times, against 24 on the merge base. MEs were bit-identical, so an ME-only comparison cannot see it. The flow lines now go to a generated ColorFlows.inc that SigmaKin.cc includes.
4db0133	(new, GPU) backend/gpu did not compile (DeviceAccessJamp2::kernelAccessIcolConst and #include "CPPProcess.h" in MatrixElementKernels.cc were dropped), and two device-semantics regressions: calculate_jamps wrote jamps with nhel = 1 while color_sum_kernel reads the [2][ncolor][nGoodHel][nevt] buffer with nGoodHel (nhel is the color stride), and the atomicAdd on the helicity-shared colAllJamp2s became a racy +=. All restored to the merge base's own lines.
e364c25	(2) Squared split orders restored instead of refused. The backend .cc files are compiled once per P* directory against its generated headers, so a compile-time switch suffices (same pattern as shouldUseBlas): ProcessData::nampso/njampso/nsqampso, pairing tables in ColorData.h, color_sum_cpu_splitorders (the old template's pair loop) under if constexpr( nampso > 1 ), jamp buffers sized njampso; GPU static_asserts nampso == 1 like the old #error. color_sum_splitorders.cc is folded in and removed.
ce3908a	(3) Drop the call to the undefined _link_backend_dirs_in_P in trex.py.

Worth changing (review items 4-6)

Commit	What
9507875	(4) Select mgOnGpuVectors.h on a MGONGPU_HAS_VECTORS_H config macro instead of __has_include, which a stray src/mgOnGpuVectors.h (TREX copies one) turned true for scalar/GPU builds. Reproduced: the stray header broke BACKEND=scalar; now it builds and matches.
0ce9731	(5) Restore MGONGPU_CPPSIMD in the two mixed-mode guards of backend/simd/color_sum.cc, so they agree with nParityCS/nParity. Latent only; output unchanged.
c501a7d	(6) Copy only regular .h/.cc files from the backend and src/rambo template dirs: a .DS_Store placed there used to land in every output.

Optional cleanup (review items 8-11, 13-14; 15 partly)

Commit	What
d76b6b2	Stop rendering process_sigmaKin_function.inc (result was discarded; it was being kept in sync as a 4th copy of SigmaKin.cc); remove it and color_sum_blas_loop.inc.
6603911	Remove get_matrix_single_process, jamp_ncolor, process_matrix.inc, color_sum_blas.inc: no caller left.
0cbc16f	Remove the empty MemoryAccessVectors.h of backend/cpu and backend/gpu.
8701fe0	backend/common/ for the 9 files byte-identical in all three backends, searched after backend/<variant>/. Deliberately no name in both: a quoted #include resolves in the including file's directory first, so a common "host" header would shadow a GPU override. The 6 files shared only by cpu and simd therefore stay duplicated.
e4cf21b	Stale comments (e.g. a CalculateJamps.cc that never existed; backend sources described as compiled once when they are compiled per P* and must be).
06b52cd	fpeEnable() moves from constexpr_math.h to its own backend/common/fpe.h.
c5db8e9	Restore the nprocesses == 1 caveat on helcolDenominators; the actual sizing is #168.

Items 7 (HRDCOD=1, already broken on main) and 12 (.gitignore PROC*) are intentionally not addressed here.

CI fixes (the 34 checks that fail on #166 as well)

Commit	What
1e93379	#include <cstring> in backend/{cpu,simd}/SigmaKin.cc and backend/gpu/MemoryBuffers.h, which call memcpy. clang/libc++ pulls it in transitively, GCC/libstdc++ does not, so on the Linux runners every madmatrix build stopped at SigmaKin.cc:71: error: 'memcpy' was not declared in this scope. That one error is 33 of the 34 red checks on #166 (every acceptancetest_mg7_*, all check_xsec_processes, acceptancetest_30/32, unittest_31). Reproduced locally with g++-16; with the fix, test_check_language_function_epem_aa, test_standalone_cpp, test_standalone_split_orders_interference and test_e_e_collision_mg7 pass under CXX=g++-16.
61201d4	FD gauge: define_gauge_dir / calculate_propagator_factor are non-template functions defined in HelAmps_<model>.h but marked INLINE, which is empty unless HELINL=1. Now that CPPProcess.h includes HelAmps and SigmaKin.cc / umami.cc include CPPProcess.h, the process library failed to link with duplicate symbols (acceptancetest_uncovered_merged, hidden behind the memcpy error on #166). Marked inline; the FD tests pass with clang and GCC and the golden is regenerated for these lines.
fb9bc7d	Regenerate the testIO_FDgauge_madmatrix golden (HelAmps_sm.h): the only differences are the two intended ones, the madmatrix namespace and the backend-conditional mgOnGpuVectors.h include.

Validation

• Against the merge base aa9d133fa, through UMAMI with identical momenta and random numbers (1024 events): g g > t t~, g g > g g g, e+ e- > mu+ mu-, MSSM u u~ > n1 n1, u u~ > u u~ QED^2==0 are bit-identical in ME, per-event selected color and selected helicity (simd_128 mixed; scalar and FPTYPE=d spot-checked).
• Split orders, FPTYPE=d: u u~ > u u~ QED^2<=4 and u u~ > d d~ g QED^2==2 bit-identical, u u~ > u u~ QED^2==2 to 9e-16; u u~ > t t~ QED^2==2 is analytically zero and noise (~1e-15) on both. In mixed precision the interference differs at the float level (~1e-6 relative), as expected from the float color sum under cancellation. At the RAMBO 1000 GeV point the interference is -5.5828746494657286e-02 against the recorded -5.5828746494657258e-02. test_standalone_split_orders_interference passes (it cannot on the current branch).
• Unit suite: 1872 tests, same two reds as on the unmodified backend_seperation_PR (scipy missing; an order-dependent MadLoop IOTest that passes alone).
• GPU: not run on hardware. No CUDA/HIP toolchain was available. Every backend/gpu translation unit was syntax-checked on the host against minimal CUDA/cuBLAS/thrust stubs, differentially: the merge base's GPU code is clean under the same stubs, backend_seperation_PR shows the two compile errors above, this branch is clean (and a split-order process hits the new static_assert). The stride and atomicAdd fixes are device semantics that check cannot see, so please build and run the cuda backend before merging.

Also noted, not fixed here

• TREX still cannot produce output, for reasons already present on main: edit_rwgt_header looks up PLUGIN_export_cpp on model_handling (trex.py defines it in its own namespace), and librex.so does not compile with Apple clang (Rex.h:1609). Candidates for a separate issue.
• Correction to the review comment on Backend separation 2 #166: it said 22 of the 30 cpu/simd file pairs were byte-identical. The actual count was 15 of 30. The recommendation does not change.

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