Custom one-shot parser GitHub issue

{"vibecode": {
    "doc": "bootstrap_example_custom_one_shot_parser",
    "role": "worked example showing the bootstrap pattern used for a one-off parser — a script processes a small custom markup format used only here, and uses `bootstrap(:parse, $source)` to construct-and-run in one expression. Demonstrates the with-method-invocation form (returns the method's return value).",
    "audience": "Caspian programmers learning the bootstrap pattern",
    "parent_doc": "index.md (bootstraps concept)"
}}

A script processes a small custom markup format used only here. The parsing logic isn't worth publishing as a class:

%vibecode
    role: 'parse a one-off markup format used only by this report script';
end

$ast = bootstrap(:parse, $source) # markup parser
    field :pos, class: 'integer', default: 0

    method &parse($input)
        @pos = 0
        # ... walk the input, build the tree ...
        # returns the parse tree
    end

    method &expect($ch)
        if @input.char_at(@pos) != $ch
            %self.error('expected ' + $ch)
        end
        @pos = @pos + 1
    end
end

Why a bootstrap. The parser has real state (position, error context, partial tree) and methods that call each other recursively. Plain functions would thread $pos, $errors, $tree through every call — noise. A class would imply "MarkupParser is a thing in the system" — but it isn't; it's just this report script's helper. The bootstrap(:parse, $source) form captures the actual intent: "parse this." The parser exists only as long as it takes to do the work.