feat: try an optional mirror before GitHub for the tree and files

[m1ngsama]

From mainland China the GitHub API often resets or times out, so the docs tree never loads; nbtca's own docs site on Cloudflare stays reachable.

• DocsClientOptions.mirror (an http(s) base URL) makes listAll and getFile try ${mirror}/index.json and ${mirror}/raw/<path> first, with a 5 s timeout, and fall back to GitHub.
• The mirror index is validated like the GitHub tree (parseTreeResponse + filterTree); a truncated or malformed index falls back. No GitHub token is ever sent to the mirror.
• A network error, timeout or 5xx stops the client asking the mirror; a 404 for one file falls back for that file only, so a document not yet mirrored doesn't cut everyone off.
• Mirrored content goes through the same sha-keyed store. Without mirror behaviour is unchanged; additive, so it ships as 0.3.3.