[LeadcodeDev]

Closes #425.

set_root_attribute kept the closing quote of the value it replaced and appended
another. An Iconify source carrying width="1em" came out as width="84"", which
does not parse, so every icon has rendered nothing since #390 — announced only by
a warning on stderr under a zero exit code.

The existing tests missed it because they assert contains("width=\"84\""), which
is true of the broken string too. The new unit test parses the rewritten SVG with
usvg instead, and two render tests go through a real scenario to pixel assertions.
Reverting the one-character fix reports the icon painted 0 coloured pixels.

render --frame did not preload at all, so an unresolvable icon warned and exited
0 there while still, sheet and the encoders exited 1. It now goes through
preload_scenario_assets like the other single-frame paths — verified at 1 and 1.

674 files in a real cache still carry the old {slug}-{colour}-{w}x{h} name. They
are recoloured copies rather than sources, but the colour is in the filename, so
the substitution is reversible: on a source miss the cache is scanned for one and
currentColor is restored from it instead of refetching. That is what lets an
offline render work from a cache filled before the rework. Parsing is anchored at
the right of the name, since an icon's own name can contain a dash
(lucide_arrow-down-8b5cf6-176x176.svg).

The fourth item — reviving validate --offline / rustmotion icons prefetch — is
not here. #320 has since landed --allow-remote-fonts, so an offline render now
has a gate for fonts and none for icons, and the shape of the icon one should
follow that decision rather than precede it. Worth its own issue.

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1822 passed.