Rebind char cycling to ,/. instead of [/]

[ and ] needed a reach/shift on most layouts and felt awkward next to
the otherwise all-unshifted control scheme (hjkl, space, digits). ,
and . are adjacent, unshifted, easy to hit without looking away from
the canvas -- a natural prev/next pair. Old bindings kept as a fallback
rather than removed.

Verified both in isolation (Update() with a KeyMsg) and live over a
real SSH session -- the status-line ink swatch visibly cycles from █
to ▓ on a fresh connection. An initial live test after the rebind
appeared to fail, but that traced to a stale session left over from
the server restart, not an actual bug -- confirmed working correctly
on a fresh connection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

game/model_test.go

@@ -118,3 +118,27 @@ func newTestRenderer() *lipgloss.Renderer {
 	r.SetColorProfile(termenv.TrueColor)
 	return r
 }
+
+func TestCommaPeriodCharCycling(t *testing.T) {
+	c := NewCanvas()
+	m := NewModel(c, "test-id", "tester", newTestRenderer())
+	c.Join("test-id", "tester", nil)
+
+	mi, _ := m.Update(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("x")}) // exit intro
+	m = mi.(Model)
+
+	t.Logf("charIdx before: %d", m.charIdx)
+	mi, _ = m.Update(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune(".")})
+	m = mi.(Model)
+	t.Logf("charIdx after '.': %d", m.charIdx)
+	if m.charIdx != 1 {
+		t.Fatalf("expected charIdx=1 after '.', got %d", m.charIdx)
+	}
+
+	mi, _ = m.Update(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune(",")})
+	m = mi.(Model)
+	t.Logf("charIdx after ',': %d", m.charIdx)
+	if m.charIdx != 0 {
+		t.Fatalf("expected charIdx=0 after ',', got %d", m.charIdx)
+	}
+}