Add host unit test infrastructure with GoogleTest

[kahrendt]

Set up an opt-in host test target (-DSENDSPIN_BUILD_TESTS=ON) that fetches GoogleTest via FetchContent and links the sendspin library, reaching its private headers for white-box testing. Add a CI job that builds and runs the suite under AddressSanitizer + UndefinedBehaviorSanitizer and gate merges on it.

Exemple tests cover the highest-value cross-platform logic:

• protocol.cpp: enum round-trips, message-type dispatch, the tri-state metadata/color deltas, and the hand-rolled client/time int64 formatter checked against snprintf (including a 20k-iteration deterministic fuzz)
• SendspinTimeFilter: monotonic-timestamp rejection, reset, offset round-trip, and convergence to a constant offset
• AudioStreamInfo: byte/frame/sample/duration conversions, including a large-frame case pinning the frames_to_microseconds overflow fix

[Copilot]

Pull request overview

This PR introduces an opt-in host-side unit test target for the sendspin C++ library, using GoogleTest and integrating it into CI so core cross-platform logic can be validated (including under ASan/UBSan).

Changes:

• Adds a sendspin_tests GoogleTest executable (enabled via -DSENDSPIN_BUILD_TESTS=ON) and a tests/ directory with initial white-box unit tests.
• Adds unit tests covering protocol parsing/formatting, the time sync filter, and audio stream conversion utilities.
• Extends GitHub Actions CI to build and run the test suite under sanitizers and gate the aggregate CI job on it.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_time_filter.cpp	Adds behavioral-invariant tests for SendspinTimeFilter (monotonic rejection, reset, convergence).
tests/test_protocol.cpp	Adds protocol-level JSON parsing/formatting tests, including round-trips and formatter oracle checks.
tests/test_audio_stream_info.cpp	Adds exact arithmetic tests for AudioStreamInfo conversions and equality behavior.
tests/README.md	Documents how to configure, build, and run the new host unit tests (including sanitizers).
tests/CMakeLists.txt	Defines the test executable, FetchContent GoogleTest setup, and CTest discovery.
CMakeLists.txt	Adds a top-level SENDSPIN_BUILD_TESTS option and hooks in the tests/ subdirectory for host builds.
.github/workflows/ci.yml	Adds a CI job that configures/builds/runs unit tests with sanitizers and gates the CI aggregator on it.

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