[fizzy-linux]

Part of fizzyedit/work#144

Checked first: main does not do this. Recording kept only the claimed agent's own commands (#6, #20, #22), so a person's or another agent's edit between two steps was invisible in the replay, and the agent's next step replayed on a document that never had it.

What changes

• Recording remembers the document as its last step left it (known). Before each step (tools.zig keyframeOthers) the document is read again; if it differs, a keyframe of it goes into the tape at that moment with a chapter <who> changed <file>. Who is the author of the top step of the document's history (Host.topStep), else "The person".
• After a step succeeds, known is the document as it left it (note, also for commands that answer later).
• write builds its tape in toTape, so it can be tested.

Verified

• New test in src/Recording.zig (zig build test: 1/1 tests passed, Build Summary 37/37): agent step, person's edit, agent step gives 2 keyframes, a chapter naming the person, ops in order, and the tape survives tape.binary encode/read; an unchanged document adds no keyframe.
• zig build of the plugin (FIZZY_PROFILE sandbox): 37/37 steps succeeded. Not run in a live fizzy.

Not done

• A failed command that partly changed the document is credited to another hand at the next step (known is only settled after success).
• The reading costs one documentBytes per step on a recorded document.

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