v0.4.0 — Playhead playback

Reworked session playback, plus new docs and the agent verification sweep.

Playback (recorded / imported sessions)

• Every frame is on screen from the start; a left-rail playhead marks the current position. Frames are no longer hidden until "played".
• Play / Pause / Replay; the scrubber starts at 0; 0.5×–8× speed.
• The timeline follows the playhead while playing (wheel to take over).
• Reload restores position from the URL: scrolls to the selected frame (seq) or the playhead (play); a finished playhead resets to the start. seq wins over play.
• Recorded and imported sessions no longer auto-jump to the bottom on open.
• Playground deep links (?url=…&seq=…&play=…) restore scroll position.

Docs & ecosystem

• docs/session-anatomy.md: how to read a capture (handshake → prompt turn → cancellation), grounded in a real WebStorm ↔ claude-agent-acp session.
• Agent verification sweep across 13 ACP agents; README "Supported editors & agents" tables with per-row status and evidence links; fixtures/drive-full-turn.mjs live smoke driver.