ADR-0004: doctest over GoogleTest

Status: accepted (2026-09)

Context

The codebase is template-heavy; test compile time matters across four CI configurations.

Decision

doctest: single header, lighter to compile than gtest (not measured here), SUBCASE fits state-machine tests, doctest_discover_tests gives per-case ctest entries.

Consequences

No gmock; we test against reference models (naive std::map books) rather than mocks.