refactor: DRY delta-upgrade, async patch-cache, fix shared-file cleanup

delta-upgrade.ts:
- Extract resolveAndApplyDelta() to unify cache-first + apply logic
  for both stable and nightly channels
- Extract resolveNightlyChainWithContext() to share GHCR setup between
  resolveNightlyDelta and prefetchNightlyPatches
- Extract prefetchAndCache() to unify both prefetch functions
- Add steps field to PatchChain and StableChainInfo so chain resolution
  directly provides version step pairs (eliminates redundant
  filterAndSortChainTags call in nightly prefetch)
- Remove buildNightlySteps() (now done inside resolveNightlyChain)
- Net reduction: 55 lines

patch-cache.ts:
- Make ensureCacheDir() async via node:fs/promises mkdir
- Parallelize patch writes in savePatchesToCache via Promise.all
- Parallelize chain meta reads in loadAllChainMetas via Promise.all
- Parallelize patch file reads in loadCachedChain via Promise.all
- Replace file.exists() checks with ENOENT handling (avoids TOCTOU)
- Make cleanup fully async with unlink from node:fs/promises
- Merge classifyChains + removeExpiredChains into single
  removeExpiredEntries with parallel deletes
- Replace all sync fs imports with async equivalents

tests:
- Add steps assertions for extractStableChain, resolveStableChain,
  and resolveNightlyChain
- Add test for shared patch file preservation during cleanup
  (validates the BugBot-reported fix)

Author: BYK