Create Reference Test Plugin and Core Test Suite #350

Objective

Build a reference test plugin and a test suite in doctest to validate the plugin engine lifecycle and hardware interactions.

Scope & Requirements

• Implement a minimal C99 plugin in tests/plugins/TestPlugin.cpp:
  • Exports plugin_query_abi_version() and valid manifest
  • Implements initialize() and shutdown()
  • Implements get_config_schema() and configure()
  • Implements a dummy slot peripheral card tracking I/O read/write counts
  • Implements on_frame_ready and on_reset event hooks
• Add a CMake shared library target for the reference plugin.
• Implement unit and integration tests in tests/unit/test_plugin_system.cpp registered in ctest.

Testing & Verification

• Build the reference plugin shared library target during test suite compilation.
• Execute ctest --test-dir build -R test-plugin-system to validate complete end-to-end plugin operations.
• Validate version handshake acceptance and rejection of mismatched ABI versions.
• Validate host bridge memory peek/poke, cycle queries, and OSD message delivery through the reference plugin.
• Validate directory scanning and manifest extraction without initialization.
• Validate event hook execution across frame, memory, and reset events.
• Validate slot insertion, I/O read/write routing, IRQ trigger, and card ejection.
• Validate passing key-value options via configure() and retrieving updated plugin state.
• Verify clean memory management across multiple load and unload cycles using memory check tools.

Acceptance Criteria

• test-plugin-system target builds and passes in ctest.
• Reference plugin compiles as an isolated shared library without warnings.
• Zero memory leaks or dangling handles detected during test runs.