Halkyon's shared packages

A pub workspace of the Dart packages Halkyon's apps share: halkyon_ui,
Halkyon's look (the theme, fonts, layout and shared widgets, with texts
in English and German), halkyon_core's low-level primitives and
halkyon_interop's cross-app contracts. Dart 3.13 with LeanCode's lints,
configured once at the root; apps take a package by git, pinned to a
signed commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/CHANGELOG.md

@@ -0,0 +1,11 @@
+## 0.1.0
+
+* Halkyon Learn's look, for every Halkyon app:
+  `appTheme(brightness, extensions: …)` with the hand-written light and dark
+  schemes around cobalt and copper, `Brand`, `ThemeExtras` (success, warning,
+  info), `PaletteColor`, `FittedSnackBars`; Noto Sans bundled
+  (`appFontFamily`, `appFontFallback`, `appFontFallbackFor`,
+  `registerFontLicenses`); `WindowSize` and `readableContentMaxWidth`; the
+  shared widgets (sheets, frames and gradients, spinners, fields, lists,
+  `BrandWordmark` and `LaunchScreen` taking an app's word and icon) with their
+  texts in English and German (`HalkyonUiLocalizations`).