feat: LLM accuracy testing framework with 17-test catalog

Framework runs a category/subcategory test catalog against multiple
LLMs through an OpenAI-compatible endpoint. Deterministic evaluators
score each response. Reports render as table, markdown, or JSON.

internal/tests/codebase_test.go

@@ -0,0 +1,64 @@
+package tests
+
+import (
+	"context"
+	"testing"
+)
+
+// TestMix_GroundTruth runs mix itself (the traced function) to ground
+// codeTraceGoWant, per PLAN.md's instruction for this specific test.
+func TestMix_GroundTruth(t *testing.T) {
+	tests := []struct {
+		name  string
+		n     int
+		depth int
+		want  int
+	}{
+		{"base case depth 0", 12, 0, 12},
+		{"one level, even", 24, 1, 13}, // 24>>1=12; mix(12,0)+1=13
+		{"two levels", 48, 2, 15},      // 48>>1=24; mix(24,1)+2=15
+		{"three levels", 96, 3, 18},    // 96>>1=48; mix(48,2)+3=18
+		{"full trace, odd start", 37, 4, 22},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := mix(tt.n, tt.depth)
+			if got != tt.want {
+				t.Errorf("mix(%d, %d) = %d, want %d", tt.n, tt.depth, got, tt.want)
+			}
+		})
+	}
+
+	if codeTraceGoWant != 22 {
+		t.Errorf("codeTraceGoWant = %d, want 22", codeTraceGoWant)
+	}
+	if got := mix(37, 4); got != codeTraceGoWant {
+		t.Errorf("mix(37, 4) = %d, want codeTraceGoWant %d", got, codeTraceGoWant)
+	}
+}
+
+func TestCodeTraceGoTest_Eval(t *testing.T) {
+	tc := codeTraceGoTest()
+
+	tests := []struct {
+		name     string
+		response string
+		want     float64
+	}{
+		{"exact correct", "22", 1},
+		{"prose wrapped", "The result is 22.", 1},
+		{"off by one", "21", 0},
+		{"forgot to track depth in the unwind", "12", 0},
+		// S6 regression: a trailing "64-bit" qualifier after the real
+		// answer must not be mistaken for the answer itself.
+		{"answer followed by a 64-bit qualifier", "The result is 22, computed via a 64-bit accumulator.", 1},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tc.Eval.Evaluate(context.Background(), tt.response)
+			if got.Value != tt.want {
+				t.Errorf("Eval.Evaluate(%q) = %v, want %v (detail: %s)", tt.response, got.Value, tt.want, got.Detail)
+			}
+		})
+	}
+}