fix chart blank: 1B was WRITING chart.png as TEXT (623-byte bogus png -> 0x0 render). write tool now REFUSES image-ext text-writes + steers to matplotlib savefig; _render_media_file skips files without a real image magic-number (no blank bubble).
fix REGRESSION: prompt hardening ('write then final answer is one sentence') made the 1B SKIP the bash run + hallucinate 'saved chart.png' without running -> no image. Restore: after writing a runnable script you MUST run it with bash + read output before answering; never claim you ran code you did not.
