refactor(api): Split CommandStore

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Background

This file has been a bit uncomfortably bloated for a while. It's responsible for at least two broad areas:

1. Run-level concerns, like getting "the overall run error," or choosing "the next command to execute."
2. Lower-level data organization concerns, to support stuff like O(1) retrieval of a command by its ID, or O(1) retrieval of the next queued command.

The other problem is that it uses the old test style of splitting the store from the view. This creates thousands of lines of tests that don't pull their weight in catching bugs, and, I find, make it harder to work on this file.

All of this has contributed to maintenance problems (here are apparently two different ways of computing the same thing, for example), and performance problems past and ongoing.

This is going to get worse because of error recovery, which will add new run states and maybe command states.

So, the plan here is:

• Pull the lower-level data organization stuff out into its own data structure. Its responsibility will be to provide O(1) access to commands, without relying on knowledge of higher-level things like runs.
• That single type will take over for at least these fields in CommandState.
• Unit-test as a pure functional black box.

Test Plan

Changelog

Review requests

Risk assessment

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Closing for now to prioritize other things.