Add self-declared test tags

[zayenz]

Summary

• add public TestTag and TestTags types plus tag-based runner selection
• make normal the default tag while requiring every non-default membership to be declared explicitly at test registration
• keep representative coverage, transcendental functions, and focused regression tests in the normal suite while moving exhaustive cross-product cases to the sweep suite
• expose matching CMake and Autoconf targets and document the installed test-harness API

Stack

This PR is stacked on #237. It targets feature/test-library so its diff contains only the test-tag layer. Review and merge #237 first; this PR can then be retargeted to main.

Validation

• cmake --build build --target check -j4
• ctest --test-dir build --output-on-failure (5/5 passed)
• clean-prefix verify-installed-test-component.py downstream consumer check
• tag inventory: 385 check, 6,299 normal, and 47,826 sweep memberships