v2: action block — return to previous modal after dispatch (explore)

## Parent

#69

## Scope: explore

Spike + design ticket. With multi-modal stacking gone (action always closes the current modal), there's still a real use case: dispatch action B from modal M1, see the result modal M2, then **return to M1**. Today that requires the user to re-trigger M1 from the resource action manually.

Open design questions:

- **Who declares the return** — the triggering block (`Block::action(...)->withReturn()`), the response (`ModalResponse::returnable()`), or implicit (always remember the previous payload until explicitly closed)?
- **State source** — client-side cache of the previous payload, or server re-dispatch of the originating action?
- **Chrome on return** — restore M1's original title/size, or whatever the return surface declares?
- **Multi-hop** — single-back only (M2 → M1) or full stack (M3 → M2 → M1)?
- **UX surface** — a "Back" button injected into modal chrome, a returnable block author-controlled in the response, or both?

## Acceptance criteria

- [ ] Design ticket — produce a follow-up implementation ticket with a chosen approach, or close this with a "won't ship" rationale.
- [ ] No code in this ticket. CONTEXT.md / ADR updates only if a decision is reached.

## Blocked by

- #70 (the new close-on-dispatch design must land first)

Start by reading parent issue #69 and blocked issue #70 to understand the modal and close-on-dispatch designs. Compare the listed return-declaration, state, chrome, multi-hop, and UX options, then document the chosen approach in a follow-up implementation ticket; if no approach is viable, record a "won't ship" rationale. Update CONTEXT.md or an ADR only if a decision is reached.

Written by the indexing model from the issue text.