tools: add rclone-based dist mirror script

[ryanaslett]

Adds tools/dist-mirror/, a replacement for the rsync endpoint on unencrypted.nodejs.org, so we can retire that server (#3960, #4085).

What it is

A ~90-line bash wrapper around rclone's HTTP backend, pointed at https://nodejs.org/dist/. It needs no Cloudflare credentials and doesn't touch R2 directly. It reads the same public listings and files everyone downloads, and the per-file HEAD checks are answered by Cloudflare's cache. Requirements: bash, rclone, jq.

• Incremental mode (default): copies release directories that are missing locally or were published in the last 14 days, then the top-level files. Version directories are copied before index.json/index.tab, so version managers never see a release listed before its files exist.
• --full: runs a complete rclone sync, which also catches changes to older releases and deletions.
• Symlinks: latest/, the latest-* directories and node-latest.tar.gz are skipped during the transfer, then recreated as real local symlinks computed from index.json. The derived map matches the release worker's latestVersions.json exactly. The roughly 70 static v0.x file aliases are mirrored as ordinary files (a few hundred MB of duplicates).
• Sends a nodejs-dist-mirror User-Agent, so we can recognise mirror traffic in our logs.

Testing

Run against production nodejs.org, with --max-size 2M so large binaries weren't downloaded:

• The 38 symlinks it created match latestVersions.json entry for entry, and they survive --full syncs.
• mtimes match upstream, and re-runs transfer nothing.
• On an up-to-date mirror, an incremental run takes about 2 s. A --full run takes about 52 s (all directory listings plus HEAD requests).
• shellcheck: clean with the repo's .shellcheckrc and with --norc.

Once this merges, I'll post usage instructions on #3960 for TUNA and the other mirrors.

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