Keep the soname symlinks of thirdparty libs in the .deb

[Fedr]

scripts/distribution.sh copied the thirdparty ./lib with cp -rL, which dereferences symlinks: libz-ng.so, libz-ng.so.2 and libz-ng.so.2.x all ship as separate regular files. The postinst ldconfig then warns for every soname file that is not a symlink, e.g.

/sbin/ldconfig.real: /usr/local/lib/MeshInspector/lib/libz.so.1 is not a symbolic link

(reported by a user in MeshInspector/MeshInspector#141 for libz, libopenvdb, libcpr, libE57Format). It also packages each such library two or three times.

• distribution.sh: the contents of ./lib are copied with cp -a ./lib/., preserving the soname symlinks inside. ./lib/. rather than ./lib because CI's ./lib is itself an absolute symlink to /usr/local/lib/meshlib-thirdparty-lib/lib - that is what -L was covering for. ./include keeps cp -rL: headers are not seen by ldconfig.
• New scripts/devops/check_deb_libs.sh <deb> <lib dir inside the package>, run by both ubuntu workflows after Extract Deb (it extracts the package into a temp dir by itself, so MeshInspector can reuse it on its own .debs): prints the .deb size and fails on absolute or dangling symlinks in the packaged lib dir, and on a regular file sitting under another library's soname (read with readelf) - exactly what ldconfig reports as "is not a symbolic link".

Size

meshlib-dev.deb, master 954b15d (release artifact size) vs this PR:

Leg	master	PR	change
ubuntu24 x64	34,916,975	32,910,616	-2.01 MB (-5.7%)
ubuntu22 arm64	30,676,303	28,664,232	-2.01 MB (-6.6%)
ubuntu24 arm64	32,515,857	30,494,976	-2.02 MB (-6.2%)
ubuntu26 arm64	33,593,364	31,183,716	-2.41 MB (-7.2%)

Verification

• Negative test: with cp -rL temporarily restored (1a23cde, reverted), the new check failed on all 7 ubuntu legs, listing libcpr.so.1, libE57Format.so.3, libopenvdb.so.10.1, libz-ng.so.2 - the same libraries as in the issue.
• With the fix, all ubuntu x64/arm64 legs are green; the C++ examples build and run against the extracted package, so the preserved symlinks resolve at link time.
• Not covered: CI never runs the real postinst (dpkg --extract only), so the absence of the ldconfig warning itself is inferred from the soname check.

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