feat(walkthrough): post-download loop — download more without restarting (#163)

[malaquiasdev]

What this PR does

After a download completes, the walkthrough no longer exits. It offers:

Done. What next?
  › Same manga — pick another volume/range
    New manga — search again
    Quit

• Same manga → re-enters at mode/range selection reusing the in-memory chapter/volume listing — no second search, no second chapter-list fetch. Grabbing vol 1, 2, 3… is now a fast sequence.
• New manga → back to title/search, keeping the already-chosen source + auth (no re-pick, no re-auth).
• Quit / Ctrl+C → clean exit with the existing cancelled/return semantics.

Why it exists

The walkthrough was one-shot: downloading another volume of the same manga meant restarting the CLI, re-typing the title, re-searching, and reloading the whole chapter list. Painful for multi-volume downloads.

How it works internally

runWalkthrough resolves source, auth (checkAuth/doRefresh), and adapter once outside an outer loop. A per-manga ChapterListingCache { chapters, volumes } holds the resolved listing. pickRange gained optional preloadedChapters/preloadedVolumes and returns { bundles, chapters?, volumes? }, fetching (listChapters/listVolumes) only when the needed slice is absent — so "same manga" reuses the cache and a mode switch (chapter↔volume) fetches only the missing listing once. "New manga" resets hit/cache to null; source/auth/adapter persist.

Partial-failure resilient: runDownloadFlow reads { failed } and emits walkthrough.download_summary_failed when failed > 0, but the loop still offers next-action.

What did not change

• executeWalkthrough signature unchanged.
• src/index.ts return handling (cancelled vs ok) unchanged; loop returns the final iteration's result (per-iteration outputs are logged by executeWalkthrough).

Test checklist

• bun test — 513 pass, 0 fail
• bun run typecheck — clean
• bun run check — clean
• Same/New/Quit prompt after download
• "Same manga" reuses cache — search/listChapters each called exactly once across iterations (asserted)
• "New manga" re-searches without re-picking source or re-auth — call counts asserted, no stale-cache leak
• Quit + Ctrl+C at the next-action prompt → cancelled/return semantics
• chapter→volume, volume→chapter, and 3-iteration chapter→volume→chapter (no cross-contamination)
• Partial failure → loop still reaches next-action + failure summary warn

Reviewer notes

• QA + inspector both APPROVED (P3 nits only): a dead-defensive title ?? "" on the same-manga branch (unreachable), minor test duplication.
• Merge conflict expected with feat(downloader): progress indicator for long-running fetches #108 (progress indicator) — both restructure src/walkthrough/index.ts. Recommend merging feat(downloader): progress indicator for long-running fetches #108 first, then rebasing this. Changes here are the loop restructure; feat(downloader): progress indicator for long-running fetches #108's are additive progress plumbing.

Closes

Closes #163

✅ Checklist

• Tested locally
• Self-review complete
• No console errors
• Code follows project conventions
• Docs updated in docs/ (if applicable)