Model failed-turn recovery as continuation turns

[roblourens]

Summary

• represent recovery as a new continuation turn adjacent to the failed turn, rather than reopening the failed turn
• mark continuation eligibility on the failed turn and introduce a continuation message that carries no new user input
• preserve the failed turn unchanged while the continuation turn records the resumed execution, response, usage, duration, and outcome
• reject continuations when another turn is active, the latest turn is not eligible, or the continuation supplies new input
• add reducer fixtures covering successful continuation, adjacency, eligibility, and invalid continuation attempts

Design tradeoff

This is the multi-turn alternative to #390. It keeps execution history append-only and makes each resumed attempt explicit in AHP state. Clients that want to present the failure and continuation as one logical interaction would need to coalesce adjacent continuation turns in their presentation model.

Scope

This is a design prototype for comparison. It updates the canonical TypeScript state, actions, reducer, and shared reducer fixtures. Generated clients, schemas, and complete specification documentation are intentionally deferred until the protocol shape is selected.

(Written by Copilot)