test: add unit and integration tests across all packages

[more-shubham]

Currently go test ./... produces no output. This is the biggest gap for production credibility.

Scope:

• internal/tracker/stats_test.go — atomic counter correctness under concurrency with -race flag
• internal/processor/engine_test.go — isSupportedFormat table-driven tests, file walking logic
• internal/processor/worker_test.go — full JPEG/PNG → WebP pipeline using synthetic 1x1 pixel generated images (no committed test data)
• internal/analytics/collector_test.go — analyzeMemoryStability, generateRecommendations, GC analysis with mock snapshots
• tests/testutil/fixtures.go — shared test helpers using t.TempDir() for zero-cleanup fixture generation

Acceptance Criteria:

• go test ./... passes
• go test -race ./... passes with zero race conditions
• No image files committed to repository
• CI pipeline runs tests on every push