Replace FtpTasks (deprecated WebRequest/SYSLIB0014) with FluentFTP

**Tier 1 — Replace.** `src/Fallout.Common/IO/FtpTasks.cs` (~83 LOC) does recursive FTP upload via the **deprecated** `WebRequest`/`WebRequestMethods.Ftp` API (`#pragma warning disable SYSLIB0014`).

### Replace with
[**FluentFTP**](https://www.nuget.org/packages/FluentFTP) — async-first, maintained, modern FTP client.

### ⚠️ Breaking change
`FtpTasks` is public API. New signatures (async, different upload entry points) break consumers → carries `breaking-change`, lands on **`experimental`**, batched to the yearly major per ADR-0004. Add a `CHANGELOG.md` entry under the next major with a migration note. Consider an `[Obsolete]` shim on the old surface if a transition path is wanted.

### Acceptance
- [ ] FTP upload backed by FluentFTP; `SYSLIB0014` suppression removed
- [ ] `breaking-change` label + CHANGELOG entry + migration note
- [ ] Targets `experimental`

_From dependency-consolidation audit, 2026-06-02._