build(release): prepare version 3.0.0

[codenameakshay]

Prepare 3.0.0 for the Material UI migration and Dart 3.12 / Flutter 3.44 SDK increase already on main. Bump the package version, align the example SDK floor, promote release notes with breaking changes, and replace the deleted skill download link with the source folder.

Preserves dependency upper bounds and existing demos. No runtime changes. This is release preparation only; nothing was published or tagged.

Validation on Flutter 3.44.4 / Dart 3.12.2:

• 135 package tests and 3 example tests pass.
• Publish dry-run passes: 0 warnings, 258 KB archive.
• Diff whitespace check passes; example lock restored.
• make check fails its formatting gate on six unchanged files: example/lib/demos/{ai_prompt_dock_demo,badges_demo,basic_tab_bar_demo,custom_transition_demo}.dart, example/lib/main.dart, and example/test/widget_test.dart.
• Package and example analysis report two existing unnecessary_underscores findings at example/lib/demos/basic_demo.dart:43 and example/lib/demos/nested_scroll_demo.dart:32.

Draft because the full quality gate remains blocked. All reported Dart files are identical to base commit 43158ee; repairs are outside this version/docs-only plan.