Release 1.0.3: Primary Rail brand

[omar-hanafy]

Summary

• launches the new Primary Rail identity with deterministic SVG masters and generated raster variants
• applies the brand across README and pub.dev, the product site, social cards, Flutter demo, PWA, Android, iOS, and macOS
• adds adaptive, round, launcher, splash, favicon, Apple touch, maskable, and GitHub social-preview assets
• releases package and assistant-plugin metadata as 1.0.3 while retaining the real autoplay product screenshot as the pub.dev thumbnail
• keeps native example projects clean on the current Flutter stable toolchain

Validation

• dart format .
• flutter analyze
• flutter test: 100 passed
• example flutter analyze and flutter test: 11 passed
• Android debug APK build
• iOS simulator debug build
• macOS debug build
• Wasm GitHub Pages build and desktop/mobile browser checks
• AI plugin validation
• flutter pub publish --dry-run: 0 warnings
• Pana 0.23.14: 160/160