v0.2.0 — Clean-prefix acquisition and trace provenance

• Truncates jump/rail acquisitions to their clean pre-failure prefix.
• Separates ledger event, outcome, and accepted.
• Adds persistent post-hoc jump, rail, freeze, and dead-trace detection.
• Adds exact ledger-based resume counting.
• Adds usable-point filename tags and min_usable_frac.
• Adds plot_usable and clean-prefix analysis helpers.
• Automatically migrates pre-0.2 ledgers and saves the original as experiment_log.txt.bak.