split Parsing\Helpers into per-domain Link / Media / Code classes

add GfmMedia and extend GfmLink with image-as-label form

- New GfmMedia parses `` with the full DokuWiki media-parameter
vocabulary in the URL slot (?100x200, ?right, ?nolink, ?recache,

add GfmMedia and extend GfmLink with image-as-label form

- New GfmMedia parses `` with the full DokuWiki media-parameter
vocabulary in the URL slot (?100x200, ?right, ?nolink, ?recache, …).
Adds `?left`/`?right`/`?center` align keywords shared with DW `{{…}}`
— gives pure-Markdown users a way to align inline images.
- GfmLink now also matches `[](target)` — the GFM equivalent
of `[[target|{{img}}]]`. Detection is post-entry, mirroring
Internallink's `^{{…}}$` check; one mode covers the whole family.
- LinkDispatch trait replaced by Helpers::classifyLink and
Helpers::parseMediaParameters — two pure static methods, shared by
DW and GFM counterparts.
- Entry patterns for GfmLink / GfmMedia simplified (permissive URL slot,
handle-time parsing), following DW's Internallink style.
- GfmSpecTest drives a test-only SpecCompatRenderer that emits bare
<img> / <a> instead of DW's wiki-wrapped HTML, recovering 13 spec
tests that previously failed/skipped only because of renderer shape.

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