feat: add --engine flag for UE version selection

[renan-renger]

Summary

• Add --engine <version> CLI flag for download and sync commands (default: UE_5.7)
• Input validation rejects malformed versions (e.g. UE_58) with clear error message
• Fallback to highest compatible version when exact match unavailable, with informational stderr message
• Interactive prompt when only higher engine versions exist — shows available versions, lets user pick or abort
• Fix pre-existing bug where bestScore = -1 prevented fallback to higher versions from working

Changes

File	What
src/api.ts	Refactor pickArtifactId → pickArtifact returning EngineResolution with match type. Split getAsset() into resolveAsset() + fetchAssetDetail(). Export ENGINE_VERSION_RE and parseEngineVersion.
src/cli.ts	Add --engine flag parsing, format validation, handleEngineResolution() with fallback messages and interactive prompt for higher-only case.

Test plan

• epic-fab download <id> --engine UE_58 → rejects with format error
• epic-fab download <id> --engine UE5.7 → rejects with format error
• epic-fab download <id> → defaults to UE_5.7 (no behavior change)
• epic-fab download <id> --engine UE_6.0 → falls back to highest available ≤ 6.0 with stderr message
• epic-fab download <id> --engine UE_5.0 (only higher available) → shows versions, prompts user
• epic-fab --help → shows --engine in Options section
• bun run typecheck → passes

🤖 Generated with Claude Code

[StarksLabs]

Merged — thank you, and I'm sorry this sat for two months.

That delay was entirely on me. I'm in school full time and this repo went quiet
while a term ran long. You did careful work and then heard nothing back, which is
the worst version of contributing to someone's project. I appreciate you not
giving up on it.

Everything in here landed:

• --engine <version> on download and sync, defaulting to UE_5.7
• format validation, so UE_58 and UE5.7 fail fast with a readable message
• fallback to the highest compatible version, with the reason on stderr
• the interactive prompt when only higher engine versions exist
• the bestScore = -1 fix — nice catch. That one was silently breaking fallback
  for everybody and nobody had noticed.

One thing worth flagging since it touched your code: #2 independently rewrote the
same paths in cmdDownload/cmdSync to add progress output, so merging both
needed a real resolution rather than a mechanical one. Your resolveAsset →
handleEngineResolution → fetchAssetDetail split is what survived — I wrapped
it in their stderr status lines instead of the other way around, because your
version is the one that can prompt. bun run typecheck is green and --engine
is verified working on the merged tree.

Thanks again for the patience.