Browser cell editing

Requested by Paul, 2026-09-16: editing text should match the cell; Cut should leave the source in place until Paste; formula entry should accept cell and range selection. This changes browser interaction, not server calculation.

• The in-cell editor inherits the cell's font, alignment, and colours.
• Cut remembers a pending move. Paste writes destinations and clears only non-overlapping source coordinates in one atomic batch. Failed paste keeps the source. Escape, another copy/cut, or leaving the window cancels the pending move. A changed source, out-of-bounds move, or a move exceeding the existing 500-write atomic limit is refused without deleting anything.
• Formula entry accepts a click or drag where an operand belongs (after =, an operator, (, or a separator), or over selected formula text after =. It inserts an A1 reference without changing the cell being edited. Enter commits normally and Escape discards the edit. The same selection works in the formula bar. References can also be typed directly.

Validation: tools/drive/editing.mjs drives the real vendored grid in Chromium with a mocked transport. It covers font inheritance, deferred and overlapping cuts, rejected/retried paste, cancellation, changed source, sheet bounds, formula range drag, formula-bar insertion, and formula cancellation. The existing deployed browser scripts have been updated for deferred Cut.

Source review: GPT-6 Astra, 2026-09-16. Findings about native text-field copy cancellation and selection of an entire existing formula were fixed and covered by regression checks. Deployed-canary review remains required before approval/merge under AGENTS.md; these component checks do not replace it.