perf(proxy): reduce nest rewrite allocations

- Dispatch shell link rewriting between streaming and buffered paths based on response size

- Reuse buffers and reduce URL construction allocations in proxy handlers

- Add nest benchmarks and align extractParts compatibility expectations with the current contract

proxy/nest_bench_test.go

@@ -0,0 +1,63 @@
+package proxy
+
+import (
+	"ghproxy/config"
+	"io"
+	"strings"
+	"testing"
+)
+
+const benchmarkInput = `
+Some text here.
+Link to be replaced: http://github.com/user/repo
+Another link: https://google.com
+And one more: http://example.com/some/path
+This should not be replaced: notalink
+End of text.
+`
+
+func BenchmarkProcessLinksStreaming(b *testing.B) {
+	cfg := &config.Config{}
+	host := "my-proxy.com"
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		input := io.NopCloser(strings.NewReader(benchmarkInput))
+		b.StartTimer()
+
+		reader, _, err := processLinksStreamingInternal(input, host, cfg, nil)
+		if err != nil {
+			b.Fatalf("processLinksStreamingInternal failed: %v", err)
+		}
+
+		if _, err = io.ReadAll(reader); err != nil {
+			b.Fatalf("failed to read from processed reader: %v", err)
+		}
+	}
+}
+
+func BenchmarkProcessLinksBuffered(b *testing.B) {
+	cfg := &config.Config{}
+	host := "my-proxy.com"
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		input := io.NopCloser(strings.NewReader(benchmarkInput))
+		b.StartTimer()
+
+		reader, _, err := processLinksBufferedInternal(input, host, cfg, nil)
+		if err != nil {
+			b.Fatalf("processLinksBufferedInternal failed: %v", err)
+		}
+
+		if _, err = io.ReadAll(reader); err != nil {
+			b.Fatalf("failed to read from processed reader: %v", err)
+		}
+	}
+}