Replace FtpTasks (deprecated WebRequest/SYSLIB0014) with FluentFTP

[ChrisonSimtian]

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 — 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.

[ChrisonSimtian]

Insights from a discarded prototype (old paths; map to onion structure after #359).

• Package: FluentFTP 54.2.0 (central).
• Async-first with sync wrappers (match HttpTasks).
• New signatures take an explicit host + server-relative paths, not full ftp:// URIs (FluentFTP separates host from paths) — this is the breaking part.
• Keep the FtpCredentials static. Remove the SYSLIB0014 suppression.
• No [Obsolete] shim: the old API parsed the host out of an ftp:// URI; preserving it would drag the deprecated WebRequest path along. Clean break on experimental is simpler.
• No FtpTasks-specific tests exist today.