[Enricoftw]

Simulink saves logged signals in a top-level group /#sigstream# of a v7.3 file. The group has no MATLAB_class attribute, so keys listed it as a variable and matread failed for the whole file, with the same error as in #128 (there for #subsystem#):

HDF5.API.H5Error: Error opening attribute MATLAB_class for object /#sigstream#

For example in MATLAB R2022b or R2026b:

ds = Simulink.SimulationData.Dataset;
ds = ds.addElement(timeseries((1:3)', (0:2)', "Name", "x"));
after = 42;
save("ds.mat", "ds", "after", "-v7.3")

#129 added #subsystem# to the filter in keys. MATLAB variable names start with a letter, so this PR skips every top-level name that starts with #. With it, ds reads as a MatlabOpaque and after as 42. The test adds a #sigstream# group to a file written by matwrite, so it needs no Simulink file.

Fix, test and text by Claude Code (Claude Opus 5.5), reviewed by Codex (GPT-6 Astra) and a second Claude Code (Claude Opus 5.5) session.