Extend EventValue with std::string and double payload types

[alexcvc]

Summary

• Extend EventValue (include/tickguard/EventValue.hpp) to std::variant<bool, std::int32_t, double, std::string>
• Add representative Catch2 coverage per new type in tests/EventDescriptorTests.cpp: initial image/snapshot, debounce send-vs-suppress, heartbeat resend, equality-driven suppression
• Add tests/EventDescriptorTypeMismatchDeathTest.cpp — a CTest death test proving the documented std::get<TValue> → std::terminate() crash contract still holds on a genuine type mismatch (EventDescriptor<bool> fed a std::string)
  • WILL_FAIL/PASS_REGULAR_EXPRESSION can't detect a signal-killed subprocess (CTest reports it as an unconditional "Exception"), so the test runs through tests/run_death_test.sh, which absorbs the signal itself and reports pass/fail via a normal exit code. POSIX-only, gated by if(UNIX) in tests/CMakeLists.txt.
• Update AGENTS.md and doc/developer-guide.md to reflect the extended variant

Test plan

• cmake --build build succeeds
• ctest --test-dir build --output-on-failure — 26/26 tests pass (was 17 before this branch of work)
• clang-format --dry-run --Werror clean
• clang-tidy clean on include/tickguard/EventValue.hpp