Add man page output for the frontends

[gmipf]

This wires up the man page reference feature added to SabreTools.CommandLine
(#2) so the frontends can produce their own man pages, instead of packagers
hand-writing and re-syncing them.

Draft: this depends on a published SabreTools.CommandLine release that
includes #2. The current pin (1.4.0) predates the man API, so CI won't
build until that release is out and the package reference here is bumped to
it. Opening now so the approach and the open questions below can be reviewed;
I'll bump the pin and mark it ready once the release is available.

What it does

• MPF.CLI man and MPF.Check man write a roff man page to standard output,
  rendered from the same command model that produces --help.
• The graphical frontend has no command-line model to render from, so a short
  hand-maintained page (MPF.Avalonia/MPF.1) is included for it.

How a man page is produced

The man feature is just another reference feature (like Help), so no new
plumbing: each program registers new Manpage(...) with its name/description
and the library renders the page from the existing feature/input model.

A packager (or anyone) then does:

MPF.CLI man > MPF.CLI.1      # 1. generate roff from the binary
# 2. stamp the .TH date at build time (see below)
# 3. install MPF.CLI.1 into the man path

Two intentional details:

• The page is emitted before the configuration path and other startup
  output, so the redirect above yields clean roff with no leading
  Configuration path: line. Generating it has no side effects: it never writes
  config.json or runs first-run setup, and the content is drawn only from the
  static command model, so the output is identical whether or not a config
  exists.
• The .TH date is left blank so a distributor stamps it at build time,
  keeping output reproducible (the library documents this). This is the only
  thing mandoc -T lint flags until it's stamped.

Installing the pages is left to distributors; this PR only adds the ability to
produce them.

Verification

• Builds clean with TreatWarningsAsErrors (spot-checked net20 and net10.0;
  full TFM sweep before merge).
• Generated pages and the GUI stub pass groff -man -ww -z with no warnings
  and mandoc -T lint clean except the intended "missing date" warning.

Open questions

• Names in .TH/NAME use the assembly names (MPF.CLI, MPF.Check, MPF).
  Happy to switch to lowercase (mpf-cli, ...) if you prefer.
• The GUI page sits at MPF.Avalonia/MPF.1; I can move it to a man/ folder.
• Detailed option descriptions are included (includeVerbose: true). Easy to
  drop to the short form if you'd rather keep the page terse.

Prepared with AI assistance (Claude Opus 4.8) and reviewed before submission.