docs: add testing documentation and macOS ARM64 known issue

Document the macOS ARM64 CGO linking issue with sqlite-vec-go-bindings
that prevents hybrid package tests from compiling locally.

Added:
- .github/TESTING.md: Comprehensive testing guide with platform-specific
  issues, workarounds, and CI configuration details
- internal/vector/hybrid/README.md: Package-specific documentation
  explaining the macOS limitation
- .github/CI_FIX_SUMMARY.md: Technical details of the CI fix

Key points:
- 41 out of 42 packages test successfully on all platforms
- hybrid package tests fail only on macOS ARM64 (local dev issue)
- Linux CI tests pass with proper build-tags: "fts5" configuration
- Production builds and runtime functionality unaffected

This is a known limitation of sqlite-vec-go-bindings on macOS ARM64
and does not impact CI/CD or production deployments.

internal/vector/hybrid/README.md

@@ -0,0 +1,32 @@
+# Hybrid Vector Storage
+
+LEANN-inspired selective vector storage with hub detection and graph-based search.
+
+## Testing
+
+### macOS ARM64 Known Issue
+
+Tests in this package currently fail to link on macOS ARM64 due to a known issue with `sqlite-vec-go-bindings/cgo`. This is a linking-time issue specific to macOS ARM64 and does not affect:
+
+- Linux CI/CD (tests pass normally)
+- Windows builds
+- Runtime functionality (only affects test compilation)
+
+The issue is caused by how macOS ARM64 handles CGO linking with multiple SQLite implementations (mattn/go-sqlite3 and sqlite-vec's embedded SQLite).
+
+**Workaround for local development:**
+- Run tests for other packages: `go test ./internal/db/... ./internal/search/...`
+- The CI on Linux will test this package
+- Or use a Linux container for local testing
+
+**Status:** This does not affect production builds or CI/CD pipelines.
+
+## Architecture
+
+This package implements three storage strategies:
+
+1. **StorageAlways** - Store all embeddings (backwards compatible)
+2. **StorageHub** - Store only frequently-accessed "hub" embeddings
+3. **StorageOnDemand** - Recompute all embeddings during search
+
+See the main documentation for more details on the LEANN algorithm and hybrid storage approach.