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/report/markdown.go

@@ -0,0 +1,77 @@
+package report
+
+import (
+	"fmt"
+	"io"
+	"math"
+	"strings"
+
+	"github.com/lukaszraczylo/llm-testbench/internal/runner"
+	"github.com/lukaszraczylo/llm-testbench/internal/testkit"
+)
+
+// renderMarkdown writes the same three tables as renderTable, formatted as
+// GitHub-flavored markdown tables.
+func renderMarkdown(w io.Writer, tests []testkit.Test, models []string, results []runner.Result) error {
+	idx := indexResults(results)
+	ew := &errWriter{w: w}
+
+	ew.println("## Per-test scores")
+	ew.println()
+	scoreHeader := append([]string{"Test", "Category", "Subcategory"}, models...)
+	writeMarkdownHeader(ew, scoreHeader)
+	for _, t := range sortedTests(tests) {
+		row := []string{t.ID, t.Category, t.Subcategory}
+		for _, model := range models {
+			r, ok := idx[t.ID][model]
+			row = append(row, cellText(r, ok))
+		}
+		writeMarkdownRow(ew, row)
+	}
+
+	ew.println()
+	ew.println("## Category rollup (mean score)")
+	ew.println()
+	catHeader := append([]string{"Category"}, models...)
+	writeMarkdownHeader(ew, catHeader)
+	for _, cm := range categoryRollup(tests, models, idx) {
+		row := []string{cm.category}
+		for _, model := range models {
+			row = append(row, meanCellText(cm.means[model]))
+		}
+		writeMarkdownRow(ew, row)
+	}
+
+	ew.println()
+	ew.println("## Model summary")
+	ew.println()
+	writeMarkdownHeader(ew, []string{"Model", "Mean", "Passed", "Partial", "Failed", "Errors", "Mean latency", "Total tokens", "Tok/s"})
+	for _, ms := range summarize(models, results) {
+		writeMarkdownRow(ew, []string{
+			ms.model,
+			meanCellText(ms.overallMean),
+			fmt.Sprintf("%d", ms.passed),
+			fmt.Sprintf("%d", ms.partial),
+			fmt.Sprintf("%d", ms.failed),
+			fmt.Sprintf("%d", ms.errors),
+			durationText(ms.meanLatency, !math.IsNaN(ms.overallMean)),
+			fmt.Sprintf("%d", ms.totalTokens),
+			rateText(ms.avgTokPerSec),
+		})
+	}
+
+	return ew.err
+}
+
+func writeMarkdownHeader(ew *errWriter, cols []string) {
+	ew.println("| " + strings.Join(cols, " | ") + " |")
+	sep := make([]string, len(cols))
+	for i := range sep {
+		sep[i] = "---"
+	}
+	ew.println("| " + strings.Join(sep, " | ") + " |")
+}
+
+func writeMarkdownRow(ew *errWriter, cols []string) {
+	ew.println("| " + strings.Join(cols, " | ") + " |")
+}