Keep matrix snapshot assertions active in Release

[luisleo526]

What changed

Re-enable assert() in test_matrix_snapshot_compat even when the Release toolchain defines NDEBUG.

Why

Copilot correctly noted on #128 that the new compatibility test's assertions became vacuous in Release builds. Debug CI exercised the assertions, so this does not change production behavior; this follow-up makes both Debug and Release gates meaningful.

Validation

• Release build of test_matrix_snapshot_compat: PASS
• Release CTest for the target: 1/1 passed
• Preprocessor check with explicit -DNDEBUG: NDEBUG is absent after processing the test translation unit

[Copilot]

Pull request overview

This PR ensures assert() checks in tests/test_matrix_snapshot_compat remain active even when the test is compiled in Release mode with -DNDEBUG, keeping the snapshot compatibility gate meaningful in both Debug and Release CI.

Changes:

• Undefines NDEBUG within test_matrix_snapshot_compat so assert() is not compiled out under Release builds.
• Adds a short comment explaining why this is necessary for the CI compatibility gate.

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