feat(html): take a sheet edit back, and tell the host what the log holds

[andiwand]

🤖 Generated with Claude Code

Step 1.4 of docs/design/spreadsheet-editing.md. Stacked on #863.

odr.editing.undo() and redo() walk the commits the page made, and ctrl/cmd+Z and ctrl/cmd+shift+Z (or ctrl+Y) reach them from the keyboard while the mode is on. An undo shows the value the op replaced and drops the op, so what the log hands out and what the page shows stay the same thing — undoing the only write into a cell leaves that position out of the envelope entirely, and undoing the second write into one leaves the first.

odr.onEditChange carries {dirty, operations, canUndo, canRedo} on every commit, undo, redo and committed(): the save button a host lights, the back-press warning it raises while the page holds unsaved edits, and the toolbar it greys. committed() is what a host calls once the file it saved matches the page again, and undo starts over from there.

test/browser/sheet/editing.html grows the log cases — 48 checks, green in Chrome.

[andiwand]

Reviewed and force-pushed, rebased on the reviewed #863. editing.html is 51 checks, the new one verified against a negative control.

A chord in another input was taken as the sheet's. pinnedKey tested the modifiers before it tested the event target, so ctrl/cmd+Z typed into the search box search_js puts on the same page undid a cell edit instead of the text just typed. The target guard sits first now, and the modifier branch reads as one thing: ctrl/cmd is the undo chord here and nothing else.

Comments cut to a line each, and the changelog entry to what a host acts on — onEditChange reports the state of the log, with the payload left to the design doc.