feat(cli): completions subcommand + package shell completions

[ractive]

Summary

Brings ff-rdp to parity with hoppy/hyalo: adds an ff-rdp completions <SHELL>
subcommand (via clap_complete, using hoppy's plural completions naming)
and ships bash/zsh/fish completion scripts inside release archives and the
deb/rpm packages built in iter-119 (#158). No man pages — explicit scope
decision, matching the hyalo/hoppy precedent.

• New Command::Completions { shell: clap_complete::Shell } subcommand,
  connection-free (writes a raw shell script to stdout, no JSON envelope).
• crates/ff-rdp-cli/src/commands/completions.rs: pure generate_to(shell, writer) core wrapped by run(shell) targeting stdout.
• Release workflow's pre-package-command now generates bash/zsh/fish
  completions on every matrix target (with a cargo run --release host-build
  fallback for cross-compiled/aarch64-pc-windows-msvc targets that can't run
  their own binary) and stages them for extra-archive-paths and the
  .deb/.rpm packaging metadata.
• crates/ff-rdp-cli/Cargo.toml's [package.metadata.deb] /
  [package.metadata.generate-rpm] tables gain three completion assets each,
  preserving the intentional zsh path asymmetry (deb: vendor-completions,
  rpm: site-functions) copied verbatim from the hyalo/hoppy precedent.

Parity rationale

hoppy already ships completions <SHELL> (plural) with bash/zsh/fish staged
into its deb/rpm via the same shared-workflow pre-package-command /
extra-archive-paths inputs. hyalo has the same feature under the singular
completion. This PR follows hoppy's plural convention per this iteration's
explicit scope. Man pages are out of scope for ff-rdp (same as hoppy's
completions-without-man baseline for CLIs that don't ship clap_mangen).

Local deb-contents evidence

$ ar p target/debian/*.deb data.tar.xz | tar tJf -
./usr/
./usr/share/
./usr/share/bash-completion/
./usr/share/bash-completion/completions/
./usr/share/bash-completion/completions/ff-rdp
./usr/share/doc/
./usr/share/doc/ff-rdp/
./usr/share/doc/ff-rdp/LICENSE
./usr/share/zsh/
./usr/share/zsh/vendor-completions/
./usr/share/zsh/vendor-completions/_ff-rdp
./usr/share/fish/
./usr/share/fish/vendor_completions.d/
./usr/share/fish/vendor_completions.d/ff-rdp.fish
./usr/share/doc/ff-rdp/README.md
./usr/share/doc/ff-rdp-cli/
./usr/share/doc/ff-rdp-cli/copyright
./usr/bin/
./usr/bin/ff-rdp

All three completion files (ff-rdp.bash, _ff-rdp, ff-rdp.fish) land at
the documented paths. cargo generate-rpm -p crates/ff-rdp-cli also built
successfully (target/generate-rpm/ff-rdp-cli-0.3.0-1.aarch64.rpm); no
rpm/rpm2cpio tooling on this macOS machine to list contents directly, but
the asset table mirrors the already-verified .deb table one-for-one.

Dry-run

Dispatched via gh workflow run release.yml --ref iter-120/completions:
run https://github.com/ractive/ff-rdp/actions/runs/29131901141

Stacking note

This PR is stacked on #158 (iter-119/linux-packages, still open) and
targets that branch, not main. It will need rebasing/retargeting onto
main once #158 merges.

Test plan

• cargo fmt clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo test --workspace -q all green
• actionlint .github/workflows/release.yml clean
• FF_RDP_LIVE_TESTS=1 cargo run -p xtask -- check-iteration-ready --plan kb/iterations/iteration-120-completions.md --base iter-119/linux-packages — 10/10 PASS
• Local cargo deb/cargo generate-rpm build with completion assets verified (see listing above)
• Dry-run workflow run completes green (in progress at time of PR creation)

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