Wikitext image keywords other than thumb are unsupported and leak into the caption

[[File:X|...]] embeds support a minimal wikitext parameter subset: a width, alt=, a caption, and thumb. The
remaining framing and layout keywords are unimplemented:

• frame — original size in a frame with a caption
• frameless — downscaled, no frame
• border — a border around the image
• alignment: left, right, center, none

Deferred from #2 and tracked in #15, which is now closed: its named follow-ups shipped in 1.2.0, and its remaining
sub-item turned out to already match wikitext. Filing this so the gap stays visible.

There is a visible symptom beyond the missing feature. FileEmbed::fromTitleAndParams() treats the last
unrecognised parameter as the caption, so an alignment keyword becomes caption text rather than being ignored:

• [[File:Cat.png|thumb|left]] renders a thumbnail captioned left, where wikitext renders a left-aligned
  thumbnail with no caption.
• [[File:Cat.png|thumb|right|Real caption]] happens to render correctly, because the real caption overwrites
  right. Parameter order hides the problem.

Low priority: nothing breaks, and it only bites when a keyword is the final parameter. Worth doing because file
embeds deliberately use wikitext syntax, so readers coming from wikitext will reasonably expect these keywords to
work — and where they are not implemented, they should at least be recognised and dropped rather than displayed.

_{AI-authored — Claude Code, Opus 4.8 (max); filed while @JeroenDeDauw asked whether #15's remaining sub-item was worth doing ahead of the 1.2.0 release; not yet human-reviewed; behaviour traced through FileEmbed::fromTitleAndParams() and core's Linker::makeImageLink(), not reproduced in a browser.}

Start in FileEmbed::fromTitleAndParams() and trace how parameters reach core's Linker::makeImageLink(). Ensure the listed framing and alignment keywords are recognized instead of becoming caption text, while preserving captions and existing width, alt, and thumb behavior; done means the final-keyword examples no longer leak keywords into captions.