Let a gesture name the manual section that shows it

The help sheet says which move does a thing, and the manual has a
picture of the thing being done, but nothing joined the two: a user
reading "Pinch it with two fingers" had no way from there to the GIF of
it.

A GESTURE tag takes an optional `manual:` field naming a heading of
docs/manual/README.md by its anchor. The scan checks every one against
the anchors the bundled page is rendered with and fails when the manual
has no such heading, so renaming a section cannot leave the sheet
linking to the top of the page; gestures-check carries the same failure
into CI. The anchor goes into gesture_book.rs as a new field, and into
docs/gestures.md as a "See it" link to manual/README.md#anchor. The help
sheet draws a "See it" button beside the title of each gesture that has
one, which opens the bundled manual at that section.

The field is additive: a tag without it is unchanged, and no gesture
carries one yet.

tools/traceability/src/main.rs

@@ -161,6 +161,15 @@ fn run_gestures(base: &Path, check: bool) -> Result<()> {
         problems.extend(p);
     }
 
+    // A `manual:` must name a section the manual has, checked against the
+    // same anchors the bundled page is rendered with.
+    let manual_source = std::fs::read_to_string(base.join(MANUAL_SOURCE))
+        .with_context(|| format!("reading {MANUAL_SOURCE}"))?;
+    problems.extend(gestures::check_manual(
+        &found,
+        &manual::anchors(&manual_source),
+    ));
+
     println!("files scanned   {}", files.len());
     println!("gestures found  {}", found.len());
     println!("places          {}", gestures::by_section(&found).len());