feat(icons): adopt Phosphor as the icon system, ship curated 80-icon set

- Restructure assets/icons: drop ui/, rename outline->regular, filled->fill, add bold/
- Pull 80 Phosphor icons in regular weight (all) + fill weight (all) + bold (status set)
- Include Phosphor MIT LICENSE
- Rewrite icons README with folder map, specs, usage examples, contribution rules

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

assets/icons/bold/spinner.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><line x1="128" y1="32" x2="128" y2="64" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="195.88" y1="60.12" x2="173.25" y2="82.75" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="224" y1="128" x2="192" y2="128" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="195.88" y1="195.88" x2="173.25" y2="173.25" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="128" y1="224" x2="128" y2="192" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="60.12" y1="195.88" x2="82.75" y2="173.25" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="32" y1="128" x2="64" y2="128" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="60.12" y1="60.12" x2="82.75" y2="82.75" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>