feat(html): mark the formula cells an edit left out of date

[andiwand]

🤖 Generated with Claude Code

Rebased on main, now that #884, #885 and #886 have landed. One commit.

Step 3.3 of docs/design/spreadsheet-editing.md.

What the markup gains

Per formula cell, in an editable render only:

<td data-odr-lock="formula"
    data-odr-formula="of:=SUM([.A1:.A2])"
    data-odr-reads="2,0,0,0,1">7</td>

data-odr-reads is sheet,first column,last column,first row,last row per
rectangle, space separated, with * for an axis a reference leaves open
(A:A). The renderer parses the cell's formula and resolves each sheet name to
the ordinal an op names it by.

What the page does with it

A commit marks the cells reading what it wrote — and the cells reading those —
with odr-sheet-stale, and raises the new odr.onCellsStale callback
({sheet, cells}). The cell still shows the result its producer cached;
nothing here recomputes one, that is step 4.

The marks follow the log, not the last write. repaintStale recomputes
them off the coalesced log after every commit, undo and redo, so an undo takes
back what it made stale and a save clears them with the log. The host hears
only when the set actually moves.

The style is a wavy underline in --odr-sheet-stale, which layers over the pin
and hover wash instead of fighting them and moves no geometry.

Why the page carries the graph

A mark has to keep up with typing, so a host round trip is the wrong shape
here. The engine's own graph (#886) is what the file side will use in step 3.4.
odr.sheet.formulaAt(column, row) hands a formula bar the expression.

formula::syntax_of(FileType) is now the one mapping from a file type to a
formula syntax; the renderer and SheetDependencies both read it.

Test

• test/src/html_test.cpp: six cases — the expression, the rectangles, a
  reference into another sheet resolving to its ordinal, the attribute
  escaping, a read-only render carrying neither, and a formula reading nothing
  stating no rectangle.
• test/browser/sheet/editing.html: 61 checks now (was 51), headless-green
  together with the other four sheet pages. The fixture gained a second formula
  cell reading the first, so the chain is checked.
• The wasm example wires onCellsStale, as the host-wiring reference.

Reference output: this changes the emitted html for every sheet holding a
formula, so compare-html stays red until the output repos get their commit
and the pins advance.

https://claude.ai/code/session_01VVmjmddv2Ui17Nptc1ggue