Keep the explorer's measurement fixture in the sandbox

The three file sizes the viewer's and the editor's limits were measured
against are built by ui-sandbox.sh now, beside the rest of the explorer's
fixture tree, so the figures in EXPLORER.md can be taken again instead of
each session inventing its own generated file and getting a slightly
different one.

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

app/ui-sandbox.sh

@@ -281,6 +281,33 @@ chmod 000 "$FILES/unreadable.txt"
 printf 'in a subdirectory\n' >"$FILES/sub/inside.txt"
 ln -sfn sub "$FILES/link-to-sub"
 ln -sfn nowhere "$FILES/broken-link"
+# The sizes the explorer's limits were measured against, so the numbers in
+# EXPLORER.md can be taken again rather than re-derived. 32k is the largest
+# the editor handles (EDIT_LIMIT); 128k is where typing loses characters;
+# 1M is FILE_LIMIT, which the viewer reads fine and the editor refuses.
+python3 - "$FILES" <<'FIXTURE'
+import pathlib, sys
+out = pathlib.Path(sys.argv[1])
+block = """/// A doc comment on function number {i}, long enough that the scanner
+/// has real comment spans to find rather than a token few.
+fn generated_{i}(input: &str, count: u32) -> String {{
+    // an ordinary line comment
+    let mut out = String::from("prefix {i}: ");
+    for index in 0..count {{
+        out.push_str(&format!("{{}}-{{}}", index, input));
+    }}
+    out
+}}
+"""
+for name, size in (("edit-32k.rs", 32 * 1024), ("edit-128k.rs", 128 * 1024),
+                   ("big-source.rs", 1024 * 1024)):
+    parts, written, i = [], 0, 0
+    while written < size - 400:
+        parts.append(block.format(i=i))
+        written += len(parts[-1])
+        i += 1
+    (out / name).write_text("".join(parts))
+FIXTURE
 fi
 
 if [ -n "$regen_config" ]; then