feat(browser): paint the page bar and the totals strip, and delegate the pointer

improvise-p7lh: `GridCache::page_coords` and `GridCache::totals` were projected
to every session and folded correctly by every client, and NO front end read
either. The shipped front page — `web/demo.improv` is `Quarter: page, Q1` —
drew a grid filtered to Q1 with nothing saying so, and `[`/`]` resolve locally
off that same `page_coords`, so a user could page through quarters watching
every number change with no way to tell which one they were looking at.

`RenderModel` gains `page_coords` and `totals`, fed straight off `state.grid`.
The Page/Filter split is asked of `PageAxis::cycle` rather than re-derived from
`items.is_some()`, which disagrees with it on an empty item list (gotcha 27).
The totals are RELAYED, never summed in the client: the sum runs over every row
including ones a viewport scrolled past (gotcha 44).

`web/tests/render_surface.test.js` gains the direction that would have caught
this when improvise-n10 added the fields. The existing guard is closed under
what `RenderModel` chose to expose, so a cache field that never reaches it is
invisible to it; the new one asks that every `pub` field of every widget cache
is read somewhere the browser runs, or is named as debt with a reason.
`grid.col_widths` is the one entry, pointing at improvise-pcje.

improvise-b8ot: the five clickable surfaces moved from one listener per node,
reallocated inside the render loops on every repaint, to one handler per stable
delegation root — the user's ruling. The operands are carried on the node as
data attributes and read back by a pure `cellAction`/`indexAction` in
`web/shell.js`, so the handler is a LOOKUP rather than a second derivation of
which data row was clicked: `gridPlan` counts data rows because a group header
is not one, and walking the table back up would disagree the moment one appears.
The editor's own keydown/blur stay direct — one transient node, one at a time,
and `blur` does not bubble.

All four documented behaviours a naive delegation loses are preserved and each
is now asserted in `harness/tests/contract.spec.js`: mousedown rather than click
(asserted by dispatching mousedown ALONE), `preventDefault` and only over a cell,
the conditionally skipped repaint (asserted by the editor's survival), and
closing a live editor before composing the click.

Gates: 1598 passed / 0 failed / 7 ignored over 37 binaries (baseline 1595);
node --test 47 over 6 files (baseline 36 over 5); Playwright 47 passed /
4 skipped (baseline 43 / 4); clippy, fmt and --check-evidence clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

web/tests/grid_plan.test.js

@@ -23,6 +23,7 @@ function grouped() {
     row_levels: 2,
     col_levels: [["Q1", ""], ["Revenue", "Cost"]],
     col_groups: [{ span: 2, label: "2026", collapsed: false }],
+    totals: [],
     editing: null,
     rows: [
       { kind: "group", label: "Hot", collapsed: false },
@@ -108,3 +109,29 @@ test("the editor is seeded from the stored value, not the drawn one", () => {
   assert.equal(plan.body[1].cells[0].text, "20,000");
   assert.equal(plan.body[1].cells[0].editText, "20000");
 });
+
+test("an empty totals strip is no strip at all, not a row of blanks", () => {
+  // Gotcha 33: an EMPTY strip means this grid has no totals — records mode,
+  // or an empty grid — so there is nothing to draw and nothing to pad.
+  assert.equal(gridPlan(grouped()).totals, null);
+});
+
+test("the totals strip is relayed, and its label spans the row headers", () => {
+  const model = grouped();
+  model.totals = [
+    { text: "9,001", numeric: true, error: false },
+    { text: "err", numeric: false, error: true },
+  ];
+  const plan = gridPlan(model);
+  assert.equal(plan.totals.span, 2, "the label spans every row-header level");
+  assert.equal(plan.totals.label, "Total");
+  assert.deepEqual(
+    plan.totals.cells.map((c) => [c.text, c.classes.numeric, c.classes.error]),
+    [
+      ["9,001", true, false],
+      ["err", false, true],
+    ],
+  );
+  // And it carries no coordinates: a total is not a cell a click can name.
+  assert.ok(!("row" in plan.totals.cells[0]));
+});