Refactor spinner lesson layout to share scroll scaffold

[MihaiCristianCondrea]

Summary

• add a reusable lesson_scroll_scaffold layout that defines the shared scroll container and show-code FAB
• split the spinner lesson UI into scaffold and content pieces so the scaffold can be reused via <include>
• update SpinnerActivity to inflate the content into the scaffold container and reference the new binding hierarchy

Testing

• ./gradlew test (fails: SDK location not configured in the container environment)

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https://chatgpt.com/codex/tasks/task_e_68e6342a3ec4832d94944388c7d27fa5

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Inflate merge-root binding without parent

Changing activity_spinner.xml to use a <merge> root removes the zero-argument ActivitySpinnerBinding.inflate(LayoutInflater) overload generated by view binding. The activity still calls ActivitySpinnerBinding.inflate(getLayoutInflater()), which will no longer compile (and even if it did, inflating a <merge> layout without a parent throws IllegalStateException: <merge> can be used only with a valid parent). The inflate call should supply a parent or use setContentView(R.layout.activity_spinner) and ActivitySpinnerBinding.bind(...) instead.

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