chore: add coverage reporting, Codecov badge, and improvement suggestions

[Copilot]

Summary

Adds infrastructure to measure and track test code coverage.

Coverage Estimate

The current test suite has ~432 individual test cases across 15 test files, covering:

Area	Source files	Status
utils/ (StrHash64, StringHelper, Utils, stox, binarytools)	~7 files, ~956 lines	✅ Well tested
events/EventSystem	1 file	✅ Well tested
audio/SoundMatrixDecoder	1 file, 284 lines	✅ Well tested
resource/archive/Archive	partial	✅ Partially tested
resource/ types, loaders, factories	partial	✅ Partially tested
utils/glob, path helpers	partial	✅ Partially tested
Fast3D interpreter + backends	~12,000 lines	❌ No tests
Controller / input mappings	~2,000 lines	❌ No tests
GUI / Window code	~3,000 lines	❌ No tests
Context, Config, ConsoleVariable	~1,200 lines	❌ No tests
Audio players, bridge functions	~1,500 lines	❌ No tests

Estimated current coverage: ~5–10% of the ~30,200 source lines (147 .cpp files).

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Changes

• .github/workflows/coverage.yml — new CI job that:

  1. Builds the test suite with -DCMAKE_CXX_FLAGS="--coverage" (gcov instrumentation via GCC)
  2. Runs ctest with dummy SDL drivers (same as test-validation)
  3. Generates an XML coverage report scoped to src/ via gcovr
  4. Uploads the report to Codecov using codecov/codecov-action@v5

  Note: A CODECOV_TOKEN repository secret must be added in GitHub → Settings → Secrets for the upload to succeed. Public repos may work without a token.

• codecov.yml — Codecov configuration that excludes tests/, build*/, cmake/, and tools/ from coverage metrics.

• README.md — Adds a live Codecov badge at the top of the file.

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Suggestions to increase coverage

The following areas would give the biggest bang for the buck:

1. Config / ConsoleVariable — Both are pure data classes with no SDL/GPU dependency. Test construction, Get/Set/Erase/Save/Load round-trips, and JSON serialization with a temporary file. These alone represent ~640 lines of testable logic.

2. Component / Part / PartList / ComponentList — The component graph (BFS traversal, Add/Remove, OnAdded/OnRemoved hooks) is already exercised in integration but has no dedicated unit tests. A thin fixture with concrete subclasses would cover hundreds of lines.

3. ResourceManager with a mock archive — Construct a ResourceManager backed by the existing in-memory TestArchive from archive_self_tests.cpp, then test LoadResource, GetCachedResource, and UnloadResource. This would cover ResourceManager.cpp, ResourceLoader.cpp, and the factory dispatch logic.

4. EventSystem edge cases — Already tested, but ListenerPriority ordering, re-entrant dispatch, and RemoveAllListeners with active iterators are missing.

5. BinaryReader / BinaryWriter endian round-trips — Add parametric tests for all primitive types across both Endianness::Big and Endianness::Little to close remaining uncovered branches.

6. StrHash64 (CRC64) length-aware interface — CRC64(data, len) overload is only lightly exercised; add tests for binary blobs containing embedded NUL bytes.

7. Resource factories (BlobFactory, JsonFactory, ShaderFactory) — These can be exercised without a running context by constructing an InitData struct and calling ReadResource on known binary fixtures.