Extract the gesture vocabulary from the code that implements it

Every gesture the application has was documented in the comment beside
the `TouchArea` that implements it. Excellent comments, and unreachable
by anyone not reading the source — which is the FR-UI-4 failure in a
different costume: a gesture nobody can find is a feature only its author
knows about.

Writing them out again in a hand-kept help page is the failure this
avoids. Two descriptions of one gesture drift, and it is always the prose
that drifts: the code is exercised every time somebody uses the
application and the page is exercised never. A help screen confidently
describing a double tap the grid stopped honouring last week is worse
than no help screen — and the grid did stop honouring one, in the commit
before this.

So the comment beside the implementation stays the only copy, and a
`GESTURE:` block beside it is scanned into two artefacts: `docs/gestures.md`
for a reader, and a Rust table for the application to draw a help sheet
from. Both committed, both gated, so neither can quietly stop describing
the code.

It lives in the traceability crate because it is the same operation on
the same input — walk the tree, pull structured tags out of comments,
render, fail if the committed artefact has moved. Only the vocabulary is
new. It scans `ui` and `apps` alone: a gesture needs an interface to be
performed on, and excluding `tools` is also what stops the scanner
extracting its own worked examples as broken gestures.

Fifteen gestures so far, across the library grid and the People screen.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/traceability/src/lib.rs

@@ -25,6 +25,8 @@ use std::path::{Path, PathBuf};
 
 use serde::Serialize;
 
+pub mod gestures;
+
 /// Requirement ID prefixes that participate in coverage.
 ///
 /// Test identifiers (UT, IT) are a separate taxonomy: they are *evidence* for