Test improvements: RF and gradient waveform accuracy

[cncastillo]

Problem

#380 asks for simulation coverage across the RF and gradient waveform constructors. The missing coverage is not just constructor coverage: the same physical waveform should simulate to the same signal across block, uniformly sampled, and time-shaped event definitions.

Solution

Adds a KomaMRICore accuracy test for the 3 x 3 RF/Grad waveform grid. Each case builds the same RF pulse and readout gradient with @addblock, samples ADC during the gradient block, and compares the signal against an OrdinaryDiffEqTsit5 Bloch reference.

The DiffEq reference path is shared through diffeq_signal, which also replaces duplicated reference code in the RF accuracy and motion tests.

Tests

• TEST_GROUP=motion julia --project=KomaMRICore -e 'import Pkg; Pkg.test()'
• julia --project=KomaMRICore -e 'import Pkg; Pkg.test()'

Development

Closes #380

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (0ab6bb9) to head (9d9cd7d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #764      +/-   ##
==========================================
- Coverage   89.62%   88.55%   -1.08%     
==========================================
  Files          76       76              
  Lines        5397     5389       -8     
==========================================
- Hits         4837     4772      -65     
- Misses        560      617      +57     

Flag	Coverage Δ
base	86.48% <ø> (ø)
core	79.43% <ø> (-8.58%)	⬇️
files	95.26% <ø> (ø)
komamri	88.32% <ø> (ø)
plots	90.88% <ø> (ø)

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see 9 files with indirect coverage changes

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