feat(html): the sheet editor formats a selection of cells

[andiwand]

🤖 Generated with Claude Code

Step 4 of the cell-formatting plan (#934): the browser side.

Change

• Selection. spreadsheet.js owns a rectangle that the pin is one corner of. A shift click, a shift arrow and a mouse drag (in the editing mode only) move the other corner. A column or row header selects its column or row across the rendered extent. odr.sheet adds selection(), select(position) and selectedCells(), and a merge answers once, at its anchor. A wider selection gets a wash of its own in both schemes.
• Formatting. odr.editing.format(style) and toggle(property) act on every selected cell, with fill (#rrggbb or null) and align (left, center, right) beside the text keys of the document editor. A lock refuses a value, not a style, so a formula cell takes a style.
• Paint. The editor patches the td and the x-p/x-s inside it with the declarations that the renderer writes. A fill also sets --odr-dark-fill, computed with a JavaScript copy of html::dark_fill, so a fill made in the dark scheme shows. In the dark scheme the renderer now also writes the light fill as --odr-fill, so the editor can read a cell's fill back.
• Host events. onSelectionChange reports the keys that the selected cells agree on. Ctrl or Cmd with B, I and U toggle, where the config gives the scripts the shortcuts.
• Log. One gesture is one undo step. The log holds one setCellStyle op per cell, the style keys of one cell merge with the later keys winning, and style ops do not mark formula cells stale.

Proof

• New test/browser/sheet/formatting.html: 57 checks, 0 failed, headless Chrome. It covers the selection gestures, a fill and its dark colour, the text keys over a direct-text cell and a styled run, alignment, undo and redo, op merging, value plus style on one cell, a formula cell, refusals, an empty style, and the chords, which leave a key typed into a field alone.
• The dark colours are pinned on both sides: html_common.a_dark_fill_is_what_the_sheet_editor_computes in C++, and the same five values in formatting.html.
• All sheet check pages pass: tests 14, positions 22, sorting 8, editing 78, keyboard 10, formatting 57.
• End to end: a real render of Senza nome 1.ods (translate CLI, editable) was formatted through odr.editing in Chrome: a 3×2 range filled and made bold, a cell italic/underlined/red/left, a cell struck at 16pt, and fills set and taken away. The op log (10 ops) was replayed with Document::edit and saved. LibreOffice renders the saved file as the page showed it.
• The full suite passes (1895 tests).

Reference output

Only resources/ changes: the three scripts and the two sheet stylesheets. Pins: public c063f3c, private a4713fd.

The last commit changes sheet-editing.js and spreadsheet.js again, so resources/ at these pins still holds the scripts before it. No page changes. A fresh run differs from the pins in those two files only, in both repositories. One more output commit and pin advance brings them in line.