Keep every retried attempt instead of deleting it

Retry used to delete the last AI action and generate a replacement, so the
discarded narration was simply gone. The row survives now: each attempt is
appended to actions.variants with variant_index naming the live one, and a
ChatGPT-style pager under the message browses them.

Action.text still mirrors the active variant, so the context builder, memory
bank, summarizer and export needed no changes. A variant carries only what
differs between attempts -- the text, the reasoning, and the state it
produced -- never the assembled prompt, which is identical across attempts of
one turn and is the bulk of context_snapshot.

Only the last message can be switched, restoring the script/world state that
attempt produced; earlier turns were written as a continuation of whatever is
active there, so theirs are read-only previews.

Three things that would otherwise bite:

- generate_turn now wraps _generate_turn and watches for a save sentinel. If
  the generator ends without it (provider error, empty reply, script stop,
  client hangup) it re-applies the previous variant -- otherwise a failed
  retry leaves rolled-back stats under un-rolled-back text.
- Retry reuses the turn's own index rather than next_index, or the clock the
  world-state cooldowns run on advances on a re-run of the same turn.
- Editing a message rewrites the active variant too, or paging away and back
  silently reverts the edit.

Migrations 34/35 verified as an upgrade against a populated database, not
just a fresh schema. test_state_revert's retry test asserted the old delete
behaviour and was rewritten.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UeQVy5bEjLhfgWNc27Efet

frontend/src/api.js

@@ -90,6 +90,14 @@ export const api = {
     request(`/adventures/${advId}/actions/${actionId}`, { method: 'PATCH', body: JSON.stringify({ text }) }),
   deleteAction: (advId, actionId) =>
     request(`/adventures/${advId}/actions/${actionId}`, { method: 'DELETE' }),
+  // Retry history. The adventure payload carries only the counts, so the
+  // attempts themselves are fetched when the reader actually pages through.
+  listVariants: (advId, actionId) =>
+    request(`/adventures/${advId}/actions/${actionId}/variants`),
+  selectVariant: (advId, actionId, index) =>
+    request(`/adventures/${advId}/actions/${actionId}/variant`, {
+      method: 'POST', body: JSON.stringify({ index }),
+    }),
 
   sendAction: (advId, payload, handlers, signal) =>
     streamSSE(`/adventures/${advId}/actions`, payload, handlers, signal),