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Add `--offline` flag to `nvm install` that resolves versions using only locally installed versions and cached downloads. No network calls are made.

New helper functions `nvm_ls_cached` and `nvm_offline_version` scan `$NVM_DIR/.cache/bin/` for previously downloaded tarballs.
In offline mode, `nvm_download_artifact` returns cached tarballs directly without checksum verification or download attempts.
The curl/wget requirement is skipped when `--offline` is set.
Supports `--lts` via locally stored LTS alias files.