view: editing an Int cell writes 5.0 #2064

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Problem

literalFor in web/view/src/main.ts turns the text typed into a cell into a literal. It treats dimension !== null as "this is a quantity".

• An Int node also has a dimension ("Int"), so typing 5 writes 5.0, which fails to type-check.
• In a table whose cells have different units, unit is null, so 5 becomes 5.0 with no unit.

Proposed fix

• Let the view model say a node's element type, or whether and how a cell is editable. Build a quantity literal only for a quantity.
• Move literalFor into a pure module and test it.