#423 Treat silent interview communication as unassessed

Summary

Prevent reports from assigning a communication score when no candidate speech was captured.

The report generator now uses the recorded candidate-turn count. Silent sessions require a null communication score and no communication strengths, while the browser displays the result as Not assessed.

Testing

• added regression coverage for silent and spoken sessions
• verified HTML and Markdown rendering
• browser tests, ESLint, Prettier, cargo fmt --check, and git diff --check pass

Closes #123.

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Summary by cubic

Stops reports from assigning a communication score when the candidate never spoke. The report generator now uses a trusted server-derived candidate-turn count; silent sessions require a null communicationScore, empty communication strengths, and display "Not assessed" in HTML and Markdown reports, while coding assessment is unchanged. Code edits and test runs are never treated as spoken responses.

• Report schema 4 makes communicationScore nullable and allows zero communication strengths.
• Interview contract bundle bumps to 40 (report prompt 24, schema 4).
• Adds regression coverage for silent and spoken sessions across prompt, validation, and rendering.