Regression: no icon renders since the icon cache rework (#365) #425

Context

Found while verifying #365 (closed by #390) on main 0.7.1. Every icon renders nothing.

Cause

set_root_attribute in crates/rustmotion-core/src/engine/renderer/assets.rs keeps the closing quote of the previous value and appends another one. Iconify sources always carry width="1em", so the output is <svg … width="84"" height="84"" …>, which does not parse. The unit tests only check contains("width=\"84\""), so they miss the stray quote.

Repro

{"version":"1.0","video":{"width":320,"height":180,"fps":30,"background":"#FFFFFF"},"scenes":[{"duration":1,"children":[{"type":"icon","icon":"lucide:rocket","style":{"width":84,"height":84,"color":"#2563EB"}}]}]}

rustmotion still -f icon.json --time 0 → failed to parse icon 'lucide:rocket': SVG data parsing failed cause expected a whitespace not '"' at 1:52, blank image, exit code 0.

To do

• Fix the attribute rewrite; add a test that parses the rewritten SVG and a render test with pixel assertions.
• render --frame only warns and exits 0 when an icon cannot be loaded, while still, render and sheet exit 1: align it.
• Old cache files named {slug}-{color}-{w}x{h}.svg are neither reused nor migrated, so an icon present many times on disk fails offline: reuse or migrate them.
• Reconsider the dropped validate --offline / rustmotion icons prefetch together with validate and still fetch Google Fonts over the network with no opt-in #320: a render that must work offline needs a way to preload its icons.

Definition of done

• A test that fails on current main is written first. For anything visual it is a render test (scenario → frame → pixel assertions); tests that build AnimatedProperties by hand are not enough — that is exactly how the regressions listed here slipped through.
• Each item is verified end to end: scenario JSON, rustmotion validate, render, image inspected.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace pass.
• Vocabulary changes update crates/rustmotion/skills/SKILL.md, the relevant rules/ file and the exported schema. A field the validator accepts must have an effect, otherwise the validator rejects it or warns.
• No comments in Rust code (repository rule). Commits follow <type>: <verb> <message>. No AI attribution in commits, PRs or issue comments.