Document fixture inline dependencies

[mavam]

🔍 Problem

• The test framework docs did not describe that project fixtures can declare
  Python dependencies inline.
• Fixture authors needed guidance for both normal test runs and standalone
  fixture mode.

🛠️ Solution

• Add how-to guidance to the fixture creation guide.
• Add reference details for scan scope, parser rules, uv installation, and
  missing-dependency behavior.

💬 Review

• Check that the guide stays task-focused while the reference page covers the
  precise behavior.

_{⚙️ Code PR: tenzir/test#47}

[github-actions]

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