Format argparse help with rich-argparse

[ajslater]

Summary

• Switch picopt's CLI to rich_argparse.RawDescriptionRichHelpFormatter (via a PicoptHelpFormatter subclass) so picopt --help is colorized.
• Add one extra highlight regex matching every registered format string (PNG, ZIP, WEBP, JPEG, …) under the metavar named group, so format names mentioned inside help text share the color of their corresponding FORMATS / EXTRA_FORMATS / CONVERT_TO metavars. The default rich-argparse highlight already handles --option references in help text.
• Rewrite the dot-color-key / doctor-mode epilog as Rich markup instead of capturing ANSI from a Console. rich-argparse renders descriptions and epilogs through console.use_theme(Theme(self.styles)), so the epilog uses [argparse.groups], [argparse.prog], and [argparse.args] to keep the section header, program name, and doctor subcommand visually consistent with the rest of the rendered help.
• The progress-char dots themselves still pull char + style straight from picopt.log.styles.MARKS, so the legend keeps its existing per-outcome colors.

Why

The previous epilog rendered Rich markup through a captured Console, then handed argparse a string of raw ANSI escape codes. That worked, but the rest of the help was uncolored, and there was no relationship between (e.g.) the FORMATS metavar color and the format names listed in the help text body. rich-argparse's highlights mechanism makes that link first-class.

Notes for reviewers

• New runtime dependency: rich-argparse~=1.8 (added to pyproject.toml / uv.lock).
• from rich.console import Console import is dropped from picopt/cli.py — Console was only used for the old ANSI-capture path.
• PicoptHelpFormatter.highlights is computed at class-definition time from registry.all_format_strs() (which is @cache'd), so there's no per-invocation cost.

Test plan

• make lint (clean)
• make ty (clean)
• pytest — 150 passed, 6 skipped
• picopt --help rendered manually; format names in help text and the epilog headers/prog/subcommand all pick up the formatter theme

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