feat: add ?? fallback support for list and string indexing

Out-of-bounds index access now throws a catchable RadPanic instead
of a fatal error, enabling fallback syntax like `list[10] ?? "default"`.

This mirrors the existing map key-not-found behavior, giving lists
and strings consistent fallback semantics with maps.

Author: amterp

core/errors.go

@@ -1,11 +1,13 @@
 package core
 
 import (
+	"github.com/amterp/rad/rts/rl"
 	ts "github.com/tree-sitter/go-tree-sitter"
 )
 
 func ErrIndexOutOfBounds(i *Interpreter, node *ts.Node, idx int64, length int64) {
-	i.errorf(node, "Index out of bounds: %d (length %d)", idx, length)
+	errVal := newRadValue(i, node, NewErrorStrf("Index out of bounds: %d (length %d)", idx, length).SetCode(rl.ErrIndexOutOfBounds))
+	i.NewRadPanic(node, errVal).Panic()
 }
 
 type RadPanic struct {

core/testing/array_test.go

@@ -152,7 +152,7 @@ a[4] = 500
 	expected := `Error at L3:3
 
   a[4] = 500
-    ^ Index out of bounds: 4 (length 4)
+    ^ Index out of bounds: 4 (length 4) (RAD20029)
 `
 	assertError(t, 1, expected)
 }
@@ -200,7 +200,7 @@ print(a[-99])
 	expected := `Error at L3:9
 
   print(a[-99])
-          ^^^ Index out of bounds: -99 (length 4)
+          ^^^ Index out of bounds: -99 (length 4) (RAD20029)
 `
 	assertError(t, 1, expected)
 }
@@ -213,7 +213,7 @@ a[-99] = 5
 	expected := `Error at L3:3
 
   a[-99] = 5
-    ^^^ Index out of bounds: -99 (length 4)
+    ^^^ Index out of bounds: -99 (length 4) (RAD20029)
 `
 	setupAndRunCode(t, script, "--color=never")
 	assertError(t, 1, expected)

core/testing/fallback_test.go

@@ -158,3 +158,80 @@ fallback
 	assertOnlyOutput(t, stdOutBuffer, expected)
 	assertNoErrors(t)
 }
+
+func Test_Fallback_ListIndex(t *testing.T) {
+	script := `
+list = [1, 2, 3]
+print(list[10] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "default\n")
+	assertNoErrors(t)
+}
+
+func Test_Fallback_ListIndexChained(t *testing.T) {
+	script := `
+list = [1, 2, 3]
+print(list[5] ?? list[10] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "default\n")
+	assertNoErrors(t)
+}
+
+func Test_Fallback_ListNegativeIndex(t *testing.T) {
+	script := `
+list = [1, 2, 3]
+print(list[-10] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "default\n")
+	assertNoErrors(t)
+}
+
+func Test_Fallback_ListIndexExists(t *testing.T) {
+	script := `
+list = [1, 2, 3]
+print(list[1] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "2\n")
+	assertNoErrors(t)
+}
+
+func Test_Fallback_ListNullValueVsOutOfBounds(t *testing.T) {
+	script := `
+list = [null, 1, 2]
+print(list[0] ?? "fallback")
+print(list[10] ?? "fallback")
+`
+	setupAndRunCode(t, script, "--color=never")
+	expected := `null
+fallback
+`
+	assertOnlyOutput(t, stdOutBuffer, expected)
+	assertNoErrors(t)
+}
+
+func Test_Fallback_ListEmpty(t *testing.T) {
+	script := `
+print([][0] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "default\n")
+	assertNoErrors(t)
+}
+
+func Test_Fallback_StringIndex(t *testing.T) {
+	script := `
+s = "hello"
+print(s[10] ?? "default")
+print(s[1] ?? "default")
+`
+	setupAndRunCode(t, script, "--color=never")
+	expected := `default
+e
+`
+	assertOnlyOutput(t, stdOutBuffer, expected)
+	assertNoErrors(t)
+}

rts/rl/errors.go

@@ -51,6 +51,7 @@ const (
 	ErrStdinRead                  = "20026"
 	ErrInvalidCheckDuration       = "20027"
 	ErrKeyNotFound                = "20028"
+	ErrIndexOutOfBounds           = "20029"
 
 	// 3xxxx Type Errors?