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/shell.test.js

@@ -147,10 +147,11 @@ test("a window smaller than its own frame still asks for one cell", () => {
 });
 
 test("the frame costs a line per column level, plus one for a group line", () => {
-  const model = (levels, groups, rowLevels) => ({
+  const model = (levels, groups, rowLevels, totals = []) => ({
     col_levels: levels,
     col_groups: groups,
     row_levels: rowLevels,
+    totals,
   });
   assert.deepEqual(frameCostOf(model([["a"]], [], 1)), { rows: 1, cols: 1 });
   // Two category levels on the column axis, and a group line above them.
@@ -158,6 +159,12 @@ test("the frame costs a line per column level, plus one for a group line", () =>
     rows: 3,
     cols: 2,
   });
+  // And the totals strip, which is chrome on the same terms as a header
+  // level: always there when there are totals, never a data row.
+  assert.deepEqual(frameCostOf(model([["a"]], [], 1, [{ text: "9" }])), {
+    rows: 2,
+    cols: 1,
+  });
 });
 
 test("the collapse glyph points right when collapsed", () => {