add GfmCode / GfmFile for fenced code blocks

GfmCode (backticks) emits the `code` handler instruction; GfmFile
(tildes) emits `file`. Column-0 fences only, no length pairing
between opener and close

add GfmCode / GfmFile for fenced code blocks

GfmCode (backticks) emits the `code` handler instruction; GfmFile
(tildes) emits `file`. Column-0 fences only, no length pairing
between opener and closer, and unclosed fences stay literal —
matching DokuWiki's `<code>` tag convention. The info string accepts
DW's full attribute vocabulary (language, filename, [options])
through a new shared `Helpers::parseCodeAttributes` that `Code`
also uses, with `html` aliased to `html4strict` and `-` meaning "no
language".

Preformatted's indent threshold is now preference-gated: 2 spaces
in DW-preferred settings, 4 spaces in MD-preferred, matching GFM's
indented code block rule. A single tab is a trigger in both.

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