Refuse an answer that would outgrow the document, and show option previews

Edits under the per-edit limit could build an answer large enough to
push the saved document past its size limit, after which the Planner
could no longer edit it. Project the answer onto a copy and validate it
first; when it does not fit, the document stays unchanged and the
question stays open.

Decisions has no preview pane, so an Atomic option's preview (a mockup
or code) now travels in its description, and people see it before
choosing without the label or the returned answer changing.

Both reported by Maggie Appleton in review.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: fail-before/pass-after passed: with about 100 KB of prose, four 50 KB edits built an answer that saved a 302 KB document past the 256 KiB limit; with the check the submit is refused, the document is unchanged and the question stays open
Assistant-verification: bun test passed: 1883 pass, 2 PostgreSQL skips, 0 fail
Assistant-verification: bun run types passed: all workspaces
Assistant-verification: bun run ci passed: dprint, oxlint, tokens, design contract, design record, Impeccable (no new findings)
Co-authored-by: Alex Lavaee <lavaman131@github.com>