feat(tui-go): render markdown in chat turns (E)

renderMarkdown wraps charmbracelet/glamour (dark style, per-width memoized) and is
hooked into the router chat-turn render path; falls back to plain content on any
glamour error so the TUI never crashes or loses text. Other roles/UI untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/markdown_test.go

@@ -0,0 +1,94 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+// stripANSI removes CSI escape sequences so we can assert on the visible text
+// glamour produced without depending on exact styling codes.
+func stripANSI(s string) string {
+	var b strings.Builder
+	for i := 0; i < len(s); {
+		if s[i] == 0x1b && i+1 < len(s) && s[i+1] == '[' {
+			j := i + 2
+			for j < len(s) && (s[j] < 0x40 || s[j] > 0x7e) {
+				j++
+			}
+			if j < len(s) {
+				j++ // consume the final byte
+			}
+			i = j
+			continue
+		}
+		b.WriteByte(s[i])
+		i++
+	}
+	return b.String()
+}
+
+// Rich markdown should be transformed (output differs from raw input) while the
+// literal words survive the round-trip.
+func TestRenderMarkdownTransformsAndPreservesText(t *testing.T) {
+	in := "# Heading\n\nSome **bold** and *italic* text.\n\n- first item\n- second item\n\n`inline code` and:\n\n```go\nfmt.Println(\"hi\")\n```\n"
+	out := renderMarkdown(in, 80)
+
+	if out == in {
+		t.Fatalf("markdown input should be rendered, got identical output")
+	}
+
+	plain := stripANSI(out)
+	for _, want := range []string{"Heading", "bold", "italic", "first item", "second item", "inline code", "fmt.Println"} {
+		if !strings.Contains(plain, want) {
+			t.Errorf("rendered output missing %q\n--- visible ---\n%s", want, plain)
+		}
+	}
+}
+
+// A plain, short string must round-trip to contain its text and not panic,
+// regardless of how much (or how little) styling glamour adds.
+func TestRenderMarkdownPlainStringRoundTrips(t *testing.T) {
+	in := "just a short plain sentence"
+	out := renderMarkdown(in, 80)
+	if !strings.Contains(stripANSI(out), in) {
+		t.Fatalf("plain string did not survive rendering: %q", stripANSI(out))
+	}
+}
+
+// Pathological / empty inputs must never panic and must return something
+// containing the input text (trivially true for empty input).
+func TestRenderMarkdownPathologicalDoesNotPanic(t *testing.T) {
+	cases := []string{
+		"",
+		"   ",
+		"\n\n\n",
+		"```unterminated code fence\nstill going",
+		strings.Repeat("a", 10_000),
+		"[](()]][[**__",
+	}
+	for _, in := range cases {
+		func() {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Fatalf("renderMarkdown panicked on %q: %v", in, r)
+				}
+			}()
+			out := renderMarkdown(in, 40)
+			// Empty/whitespace-only inputs fall back to the original content.
+			if strings.TrimSpace(in) == "" {
+				if out != in {
+					t.Errorf("whitespace input %q should fall back to itself, got %q", in, out)
+				}
+			}
+		}()
+	}
+}
+
+// A non-positive width must not crash and should fall back to the default
+// wrap width rather than feeding glamour an invalid value.
+func TestRenderMarkdownZeroWidthFallsBack(t *testing.T) {
+	out := renderMarkdown("**hello** world", 0)
+	if !strings.Contains(stripANSI(out), "hello") {
+		t.Fatalf("zero-width render dropped text: %q", stripANSI(out))
+	}
+}