refactor(routing): clarify Floor/Ceil semantics + extend test coverage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/routing_test.go

@@ -29,6 +29,7 @@ func TestLoadRoutingDefaults(t *testing.T) {
 	assert.InDelta(t, 0.90, cfg.RouteLocalFloor, 1e-9)
 	assert.InDelta(t, 0.70, cfg.RouteLocalCeil, 1e-9)
 	assert.Equal(t, 60, cfg.PassRateTTLSeconds)
+	assert.Equal(t, "", cfg.LiteLLMAPIKey)
 }
 
 func TestLoadRoutingFromEnv(t *testing.T) {
@@ -63,3 +64,10 @@ func TestLoadRoutingRejectsBadFloat(t *testing.T) {
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "HYPERGUILD_ROUTE_LOCAL_FLOOR")
 }
+
+func TestLoadRoutingRejectsBadInt(t *testing.T) {
+	t.Setenv("HYPERGUILD_PASS_RATE_TTL_SECONDS", "not-a-number")
+	_, err := config.LoadRouting()
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "HYPERGUILD_PASS_RATE_TTL_SECONDS")
+}