meson: remove Python from default build path

[Alb3e3]

Addresses #4641.

This removes the Python helper scripts from the default Meson setup/install path:

• replaces the setup-time GetZstdLibraryVersion.py call with Meson DSL and a consistency assertion against lib/zstd.h
• replaces the InstallSymlink.py install hooks with Meson's native install_symlink()
• deletes the now-unused default-path helper scripts

The test-only build/meson/tests/valgrindTest.py remains unchanged, so Python is still only needed for that explicit test path.

Validation:

• git diff --check HEAD~1..HEAD
• default-path helper guard:
  rg -n "GetZstdLibraryVersion\\.py|InstallSymlink\\.py" build/meson/meson.build build/meson/programs/meson.build
  returned no matches
• Meson 1.10.1 + Ninja 1.13.2:
  meson setup /tmp/zstd-meson-final-build build/meson --prefix=/tmp/zstd-meson-final-install -Dbin_tests=false -Dbin_contrib=false
ninja -C /tmp/zstd-meson-final-build
meson install -C /tmp/zstd-meson-final-build
• Muon 0.5.0 with no python or python3 visible in PATH:
  muon-meson -C build/meson setup -Dprefix=/tmp/zstd-muon-final-install -Dbin_tests=false -Dbin_contrib=false /tmp/zstd-muon-final-build
ninja -C /tmp/zstd-muon-final-build
muon-meson -C /tmp/zstd-muon-final-build install
• Samurai backend smoke on the Muon-generated build.ninja:
  samu -C /tmp/zstd-muon-final-build -t clean
samu -C /tmp/zstd-muon-final-build
• make -j2 zstd
• zstd CLI compression/decompression round trip with cmp

The installed alias checks confirmed:

• bin/zstdcat -> zstd
• share/man/man1/zstdcat.1 -> zstd.1