v0.2.3

Implements Synod 0.2, now complete. synod run draws the run: next to the
trace it writes scene.json, the animation the player will show. Programs
without their own look are drawn by the default look: node cards with their
state, messages flying between nodes, crashes, broken invariants and
captions.

Added

• synod run writes out/scene.json (synod-scene/1) next to the trace and prints scene <length>, <n> shapes -> out/scene.json (the length in seconds, such as 147.98s) after the existing status lines. The scene is written for every run, also after sim.Failed; --summary prints the same text as before and writes the scene silently.
• --scene file (- for stdout, with the status lines on stderr; --trace - and --scene - together exit 2), --pace x (0.25 to 4, up to three decimals) and --mode ordered|proportional. --watch and --json-events still exit 2.
• The scene builder runs the observers of the program's looks over the facts of the run. A program that does not use std/look is drawn by std/look/basic: node cards, state rows, message flights, the sim.Broken border and lesson captions.
• A fault inside a look is a look failure, not part of the trace: look failed at <time>: <pos>: error[<code>]: <message>, exit 1, and a partial scene with a failed marker.
• trace.Decode reads trace.jsonl back into facts; Build(facts) equals Build(Decode(Encode(facts))).
• std: look.Flight, net.Id, net.Type, sim.StateRows, and look.Tween of "x" and "y" to a shape; look.Travel, look.Remove and std/look/basic are ordinary Synod.
• Golden scene.json for every example, scene determinism over 20 seeds, scene hashes in the cross-OS CI job, FuzzScene in the weekly fuzz run, and BenchmarkScenePing5000.
• The run fuzz target and the generator now also build scenes, with generated looks.

Changed

• The status of a failed run is followed by the scene line, so the rich failed at … goldens of testdata/run/errors end with it.