feat: NucleusIcons

[Priva28]

Summary

Adds icon tokens to nucleus. Source SVGs live in tokens/definitions/icons/{outline,regular,solid} (140-ish per variant, 141 unique icons total — most ship in all three variants, a handful are variant-specific). The build pipeline now emits per-platform icon outputs alongside the existing colors and fonts.

Almost all of it is auto-generated icon assets. Per icon, per variant we emit:

• iOS: an .imageset directory with Contents.json + the SVG (≈ 832 files)
• Android: a vector drawable XML (≈ 416 files)
• Web: the raw SVG copy (≈ 416 files, only if build/ is tracked)

iOS

• New NucleusIcons SPM library with Resources/Icons.xcassets (one .imageset per icon × variant, all template-rendered with vector preservation).
• NucleusIcon is a String-rawValue enum where the raw value is the kebab-case icon name. .image(_:) and .uiImage(_:) return a SwiftUI Image? / UIImage? for a given Variant, returning nil if the icon doesn't ship in that variant. availableVariants exposes which variants exist per case.
• API is intentionally minimal/raw for now.

Android

• 416 vector drawables generated from the SVGs and dropped into android/nucleus/src/main/res/drawable/ with a nucleus_icon_ prefix.
• NucleusIcons.kt is an enum class with nullable @DrawableRes fields per variant, plus drawableRes(variant) and @Composable painter(variant). We tint at the call site (Icon(painter, tint = …)) — drawables ship with a black placeholder fill.
• Wrote a tiny SVG → VectorDrawable converter in tokens/formats/icons-android.ts. Most icons are pure paths, but gift.svg uses <rect>/<circle>/matrix transforms, so the converter handles those by flattening to path data.

Web

• All SVGs copied to build/web/icons/{name}-{variant}.svg.
• New build/web/nucleus-icons.json manifest indexed by icon name with both file paths and inlined SVG strings per variant.
• package.json template updated to ship icons/*.