CB-WP-0016: the drop target that was never there

Provenance (tier S, one paragraph in lieu of survey and ADR): the human
check that kept INTENT stage 1 open was run and the drag was broken.
Root cause, worth more than the instance: drop targets were ids, and an
id must be unique, so exactly one element could ever be seat-0. The
relationship-graph circle took it and the seat card that every action
card's own text points at -- 'drag Attack onto a seat' -- silently had
none. A seat is drawn twice and both drawings are the seat; the document
model could not express that.

Drop keys are now data-drop. Any number of elements may carry the same
key, so a seat is droppable on its card and on its graph node. Measured
on a live server: seat-0/1/2 each appear twice, id survives only on
cb-status which is the one element the script looks up, and
down=action-attack&up=seat-1 returns ok.

Second defect: a drop on nothing returned without posting and without
touching the status line, so a broken target was indistinguishable from
a working page. resolve already refuses rather than defaulting, which is
right; refusing SILENTLY is not. The page now reports the raw fact --
'took action-attack, let go over nothing droppable' -- which names
elements, not moves, so ADR-0007 control 5 holds.

And the honest part: the general check added here -- every offered
affordance names a key that exists, driven through Policy::choose over
four real bot games -- does NOT catch the reported defect. seat-0 did
exist, on the graph circle. It is kept because a wholly absent target is
a real class, and paired with a targeted regression test that does catch
it. Three mutations, each red for its stated reason, including the
reported defect reintroduced; only the targeted test fires on that one.

A cb-play assertion matched id="action-ground" as a substring while
describing itself as checking the page; rewritten through drop_keys.

make all exits 0. Stage 1 stays open: verified by tests, mutation and a
live server, not by a human dragging.

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

tools/cb-play/src/hotseat.rs

@@ -295,9 +295,13 @@ mod tests {
         let replies = client.join().expect("client thread");
         assert!(replies[0].contains("200 OK"), "{}", replies[0]);
         assert!(replies[0].contains("GROUND"), "the page was not the table");
+        // Through the parser, not a raw attribute match: this assertion
+        // read `id="action-ground"` and CB-WP-0016 moved drop keys to
+        // `data-drop`, so a substring test drifts silently on the next
+        // rename while still describing itself as checking the page.
         assert!(
-            replies[0].contains("id=\"action-ground\""),
-            "the offered action was not on the page"
+            cb_render_html::doc::drop_keys(&replies[0]).contains("action-ground"),
+            "the offered action was not a drop target on the page"
         );
         assert!(replies[1].contains("200 OK"));
         assert!(server.refusals().is_empty());