refactor: вынести подготовку прогона из _collect_one

[axisrow]

Summary

• extract resume validation and fresh-run manifest creation into _prepare_run
• return explicit _PreparedRun state to _collect_one
• preserve resume mismatch, no-overwrite, stale-entry pruning, and manifest write behavior

Closes #44

Tests

• ruff check .
• pytest -q (190 passed)
• mutation checks: resume phrase mismatch (red when validation bypassed), initial manifest write (red when removed); an exploratory complete-run guard mutation was not killed because the selected existing tests do not exercise that helper branch.

No tests changed.

[axisrow]

Changes requested. The fresh-run preparation regresses manifest provenance: source_url is captured before the requested phrase is entered, so this PR is not ready until it is captured after _set_phrase (and region setup) or refreshed before the first manifest write. No other high-confidence findings.

[axisrow]

Changes requested. The URL regression is fixed, but fresh-run updated_at still does not follow the manifest timestamp contract. No other high-confidence findings.

[axisrow]

Approved. The current head fixes the fresh-run URL provenance issue and now timestamps the post-setup manifest write with updated_at=datetime.now(UTC). I found no remaining high-confidence correctness, security, or error-handling issues in the diff.