Operators can replicate multiple local→remote pairs in one invocation

## User story

As an **operator with multiple pools or multiple destination roots**, I want to **describe several `LOCAL_FS → REMOTE_FS` pairs in a single `zfs-replicate` run** so that **my scheduler runs one unit of work instead of a shell loop around the CLI**.

## Why

Today the CLI takes a single `(HOST, REMOTE_FS, LOCAL_FS)` triple. NixOS packaging bolts external loops around it. #92 (2020) has been open for this. Supporting repeated `--pair LOCAL:REMOTE` (keeping the current positional form for back-compat) removes the external loop without breaking existing invocations.

## Acceptance criteria

- [ ] A new repeatable CLI flag `--pair LOCAL_FS:REMOTE_FS` exists.
- [ ] The existing positional `HOST REMOTE_FS LOCAL_FS` form continues to work unchanged.
- [ ] When `--pair` is given, each pair runs against the supplied `HOST` with the shared SSH/compression/send/receive options.
- [ ] Pairs run in a deterministic order when `--jobs 1`; run concurrently when `--jobs > 1`.
- [ ] Exit code is non-zero if any pair fails; successful pairs still complete.
- [ ] `--help` documents the flag and gives an example.
- [ ] README gains a "Replicating multiple datasets" section.
- [ ] CHANGELOG entry under "Added".

## Out of scope

- Per-pair overrides for compression/cipher/options — tracked in the config-file issue.

## Notes

- Target release: **v4.4.0** (minor — additive flag; positional form preserved)
- Closes the user need from #92.
- Pairs naturally with `-j/--jobs`.
- Source: modernization assessment §3.6.