Show the bounds check failing, because "checked" without a message says little

The claim worth making is not that there is a check but that a failure names
the line, and the only way to show that is to trip one.

web/examples/bounds.flan

@@ -0,0 +1,10 @@
+(defconst xs [3 i32] [1 2 3])
+
+;; (at xs 7) with a literal index does not reach the backend at all: check.ml
+;; rejects it. This one goes through a local, so it is the runtime check that
+;; catches it — the same message, and the program stops where it happened.
+(defn main []
+  (let [i 7]
+    (print-line "before")
+    (print-i64 (i64 (at xs i)))
+    (print-line "unreachable")))

web/examples/bounds.out

@@ -0,0 +1,3 @@
+before
+bounds.flan:9:28: index 7 is out of bounds for length 3
+exit 134

web/index.html

@@ -294,10 +294,28 @@ is why <code>(set (.hp p) 8)</code> above is legal when <code>p</code> is a
 
 <h3>Bounds are checked</h3>
 
+<pre><code>(defconst xs [3 i32] [1 2 3])
+
+;; (at xs 7) with a literal index does not reach the backend at all: check.ml
+;; rejects it. This one goes through a local, so it is the runtime check that
+;; catches it — the same message, and the program stops where it happened.
+(defn main []
+  (let [i 7]
+    (print-line "before")
+    (print-i64 (i64 (at xs i)))
+    (print-line "unreachable")))</code></pre>
+
+<pre><code class="sh">$ flan run bounds.flan
+before
+bounds.flan:9:28: index 7 is out of bounds for length 3
+$ echo $?
+134</code></pre>
+
 <p><code>at</code> and <code>slice</code> emit a comparison and a branch to a cold
-block that names the source location and stops. A literal index out of bounds is
+block that names the source location and stops. Checks are on by default and are not
-rejected at compile time instead. Checks are on by default and are not tied to the
+tied to the optimisation level, which is what lets the acceptance table run the same
-optimisation level; <code>--no-bounds-checks</code> turns them off. Measured cost on a
+programs at <code>-O0</code> and <code>-O2</code> with identical checks;
+<code>--no-bounds-checks</code> turns them off. Measured cost on a
 50-million-iteration dependency chain over a 1024-element array: 0.11–0.12s checked
 against 0.12–0.13s unchecked.</p>