fix: clear remaining staticcheck + go mod tidy findings

Static Analysis findings (all real, all fixed in place):
- filter/warmup.go: rename receiver pc→p to match the other 33 methods
  on PipelineCoordinator (ST1016)
- simd/simd.go:207: bytes.Index(x, y) >= 0 → bytes.Contains(x, y) (S1003)
- logging/logger_test.go:31: context.WithValue with string key → define
  a local ctxKey string type (SA1029 — prevents cross-package key
  collisions; the whole point of that lint)

go mod tidy: go.mod + go.sum had drifted. Lint 'Check go mod tidy' diff-
checks after tidying, which was failing on stale indirect entries.

Coverage, Import Check, Benchmark passed in the previous run; remaining
failures all from the Static Analysis + go-mod-tidy steps.

Author: Patel230

go.mod

@@ -3,9 +3,9 @@ package filter
 import "sync/atomic"
 
 // Warmup pre-initializes pipeline components
-func (pc *PipelineCoordinator) Warmup() {
+func (p *PipelineCoordinator) Warmup() {
 	dummy := "warmup test data"
-	pc.Process(dummy)
+	p.Process(dummy)
 }
 
 // LockFreeCounter atomic counter

go.sum

@@ -27,8 +27,9 @@ func TestLogger_WithContext(t *testing.T) {
 		t.Error("expected non-nil logger")
 	}
 
-	// Test with trace ID
-	ctx = context.WithValue(context.Background(), "trace_id", "abc-123")
+	// Test with trace ID (typed key per SA1029)
+	type ctxKey string
+	ctx = context.WithValue(context.Background(), ctxKey("trace_id"), "abc-123")
 	newLogger = logger.WithContext(ctx)
 	if newLogger == nil {
 		t.Error("expected non-nil logger with trace_id")

internal/filter/warmup.go

@@ -204,7 +204,7 @@ func FastContains(s, substr string) bool {
 
 	// For small patterns, use bytes.Index
 	if len(substr) <= 16 {
-		return bytes.Index([]byte(s), []byte(substr)) >= 0
+		return bytes.Contains([]byte(s), []byte(substr))
 	}
 	return strings.Contains(s, substr)
 }