feat(wasm): edit and save a sheet from the example

[andiwand]

🤖 Generated with Claude Code

Step 1.5 of docs/design/spreadsheet-editing.md, which closes step 1. Stacked on #865.

The example gains an edit and a save button. It assigns the three callbacks on the frame's contentWindow once it has loaded — the same moment droid and ios go in through evaluateJavascript — lights the save button off onEditChange (with the op count on it), and puts a refusal in the status line, which is where an app would raise a snackbar in its own string catalogue.

A view holds its own log (decision 6), so the example writes the log into the document when the view goes away as well as before a save; a cell op is idempotent, so doing both is safe, and re-rendering the view shows what was written.

edit.test.mjs writes a cell by position through the bindings and reopens what was saved, on an .ods built in memory beside the .odt — the binding-level pin for setCell.

Verified by hand: drop ods/file_example_ODS_10.ods into the example, type 777 into B2, save, drop the saved bytes back in — the reopened sheet shows 777, and the save button falls back to save… when committed() runs. ctest --test-dir build-wasm/wasm is green (43 tests).

[andiwand]

Reviewed and force-pushed.

A refused op was swallowed, and the file downloaded anyway. The page has no lock for a position the engine cannot write yet — an .xlsx cell with no <c>, an .ods one with no element — so doc.edit throws where the page took the edit happily. collect() catches it, puts the message in the status line and returns false, and the save stops there rather than handing over a file without the edits; the log is kept, so nothing typed is lost. Verified live: typing into A11 of xlsx/file_example_XLSX_10.xlsx now says UnsupportedOperation — unsupported operation, downloads nothing and keeps its op, while the .ods round trip still saves and reopens with the edit in it. Noted in the design doc's complications, since it goes away with step 2.

Conciseness:

• the changelog entry is gone — an example page is not API, formats, rendering, behaviour or packaging, which is what the file says an entry is for;
• minimalOdt and minimalOds were the same package with a different content.xml, so the mimetype, manifest and zip call sit in one odf() wrapper — 30 lines shorter than the copy this PR started with;
• the wasm test dropped the two markup assertions (html_test.cpp pins those) for the assertion it was missing: the edit renders before it is saved;
• #edit[hidden], #save[hidden] { display: none } was dead — only #drop needs it, having a display of its own;
• let name shadowed window.name; it is filename.