perf(js): use native search when String.contains needs no packed-Char walk

bend2/comp.ts

@@ -529,6 +529,10 @@ function char_code(s) {
 }
 
 function string_contains(s, p) {
+  // Without a packed-Char marker, native substring search respects boundaries.
+  if (!s.includes("\uDC00")) {
+    return s.includes(p);
+  }
   // A match must start at a Char boundary, not inside a packed U32 code.
   for (let at = 0; at <= s.length; at += char_len(s, at)) {
     if (s.startsWith(p, at)) {

guide/GUIDE.md

@@ -488,8 +488,8 @@ def main() -> String:
   U32.show(a) ++ " = " ++ Nat.show(b)
 ```
 
-Every def is named `Type.verb`, and the same verbs recur across `Nat`, `U32`
-and `F32`: `add sub mul div mod` for arithmetic, `and or xor not shl shr` for
+Names follow `Type.verb`; `Nat`, `U32` and `F32` share verbs:
+`add sub mul div mod` for arithmetic, `and or xor not shl shr` for
 bits (`U32` only), `cmp` (returning `Cmp`, not on `F32`) and `is_eq is_ne is_lt
 is_le is_gt is_ge` (returning `Bool`) for comparisons, `show` to `String` and
 `read` back from it (answering a `Maybe`). Operators and `<`-style comparisons
@@ -500,6 +500,8 @@ result), `Set` on top of it, the `Equal` lemmas, and the effects. `bend base`
 prints all of it, `bend base --types` only the types, and `bend base Map` one
 name and everything under it.
 
+String searches respect Char boundaries.
+
 ### Modules
 
 A module is a file, and an import gives it a local name:

tests/base/string_contains_boundaries.bend

@@ -0,0 +1,22 @@
+import Base
+
+def packed(n: U32) -> String:
+  SCon{Char.from_u32(n), ""}
+
+def main() -> IO(Unit):
+  do IO<Unit>:
+    IO.print(Bool.show(String.contains("abc", "")))
+    IO.print(Bool.show(String.contains("hello", "ell")))
+    IO.print(Bool.show(String.contains("hello", "x")))
+    IO.print(Bool.show(String.contains("a\u{1F600}b", "\u{1F600}")))
+    IO.print(Bool.show(String.contains(packed(1114177), "A")))
+    IO.print(Bool.show(String.contains("\u{10000}abc", packed(407048291))))
+    IO.print(Bool.show(String.contains(SCon{Char.from_u32(1114177), "A"}, "A")))
+
+#|True
+#|True
+#|False
+#|True
+#|False
+#|False
+#|True