feat(captcha): self-consistency voting, optimized prompts, no-targets signal

- Add Samples to TextOptions/GridOptions; N>1 runs N solves at temp 0.5
  and majority-votes (text: most frequent; grid: tiles in strict majority).
  N<=1 keeps the single deterministic call path unchanged.
- Add NoTargets to GridResult so callers know to click Verify instead of
  looping; prompt instructs empty tiles + no_targets:true when nothing matches.
- Grid prompt: numbered cells 0..R*C-1, select any partial overlap.
- Text prompt: disambiguate similar glyphs (0/O 1/l/I 5/S 2/Z 8/B).
- Rotation prompt: reason step-by-step then emit final JSON.

internal/captcha/grid_test.go

@@ -1,12 +1,32 @@
 package captcha
 
 import (
+	"bytes"
 	"context"
+	"image"
+	"image/color"
+	"image/png"
 	"testing"
 
 	"github.com/lukaszraczylo/captcha-solver-mcp/internal/llm"
 )
 
+// smallPNG returns a tiny valid PNG for exercising preprocessing paths.
+func smallPNG(t *testing.T) []byte {
+	t.Helper()
+	img := image.NewRGBA(image.Rect(0, 0, 6, 6))
+	for y := 0; y < 6; y++ {
+		for x := 0; x < 6; x++ {
+			img.Set(x, y, color.RGBA{R: uint8(x * 40), G: uint8(y * 40), B: 80, A: 255})
+		}
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, img); err != nil {
+		t.Fatalf("encode png: %v", err)
+	}
+	return buf.Bytes()
+}
+
 func TestSolveGrid(t *testing.T) {
 	f := &llm.Fake{VisionResp: `{"tiles":[0,4,8],"confidence":0.8,"recheck_recommended":true}`}
 	res, err := SolveGrid(context.Background(), f, []byte{1}, "image/png", GridOptions{
@@ -25,3 +45,53 @@ func TestSolveGrid(t *testing.T) {
 		t.Fatal("recheck flag lost")
 	}
 }
+
+func TestSolveGridVoting(t *testing.T) {
+	f := &llm.Fake{VisionResponses: []string{
+		`{"tiles":[0,1],"confidence":0.9}`,
+		`{"tiles":[0,2],"confidence":0.9}`,
+		`{"tiles":[0,1],"confidence":0.9}`,
+	}}
+	res, err := SolveGrid(context.Background(), f, []byte{1}, "image/png", GridOptions{
+		Instruction: "select all buses", Rows: 3, Cols: 3, Samples: 3,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	// 0 in 3/3, 1 in 2/3 (>half), 2 in 1/3 (dropped).
+	if len(res.Tiles) != 2 || res.Tiles[0] != 0 || res.Tiles[1] != 1 {
+		t.Fatalf("voted tiles %+v, want [0 1]", res.Tiles)
+	}
+	if len(f.VisionCalls) != 3 {
+		t.Fatalf("expected 3 vision calls, got %d", len(f.VisionCalls))
+	}
+}
+
+func TestSolveGridNoTargets(t *testing.T) {
+	f := &llm.Fake{VisionResp: `{"tiles":[],"confidence":0.9,"no_targets":true}`}
+	res, err := SolveGrid(context.Background(), f, []byte{1}, "image/png", GridOptions{
+		Instruction: "select all buses", Rows: 3, Cols: 3,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !res.NoTargets {
+		t.Fatal("no_targets flag lost")
+	}
+	if len(res.Tiles) != 0 {
+		t.Fatalf("expected empty tiles, got %+v", res.Tiles)
+	}
+}
+
+func TestSolveGridPreprocessing(t *testing.T) {
+	f := &llm.Fake{VisionResp: `{"tiles":[0,4],"confidence":0.8}`}
+	res, err := SolveGrid(context.Background(), f, smallPNG(t), "image/png", GridOptions{
+		Instruction: "select all buses", Rows: 3, Cols: 3, Annotate: true, Upscale: 2,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.Tiles) != 2 {
+		t.Fatalf("expected 2 tiles, got %+v", res.Tiles)
+	}
+}