fix: a State belongs to one run, not to the Pipeline

[JoshuaWink]

What

A State belongs to one run, not to the Pipeline. Closes #11.

Before this change, two runs of one Pipeline that were in progress at the same time wrote into one State, so their steps mixed.

The rule (conformance/API.md, "State")

• Each run writes into a State of its own.
• The caller can give the State: run(payload, state). A caller with more than one run at a time gives each run its own.
• With no State from the caller, the run makes a new one.
• state on the Pipeline is the State of the run that started last. It is for a caller with one run at a time.

Nothing breaks for a caller with one run at a time: run(payload) and pipeline.state work as before.

Changes

Python

• run and run_async take an optional state. The State is a local of the run.
• A second fault of the same kind is fixed. A Valve kept "was it skipped" on itself, and each run in progress shared that flag. A run that waited on an async Filter behind a Valve could read the answer of another run. The Pipeline now asks the Predicate itself and reads no flag from the Valve.

TypeScript

• run takes an optional state. Each run already wrote into its own State; a caller had no way to get it when runs overlapped.

Conformance

• Three new cases in cases/state.json: the caller gives the State, a run that starts inside another run has its own State, and state on the Pipeline is the State of the run that started last.
• A new reference step, run_again, starts a second run of its own pipeline inside the first one. This makes two runs overlap in the same way in each runtime, with no threads and no timers.
• The step does not hold a Pipeline. The runner gives it the way to run the pipeline again: a method of the Pipeline, or for a compiled form a second call of its function. So both compilers lower the step, and no case is unsupported on any target.
• One check does not apply to a compiled form: state on the Pipeline object. A compiled form is a function and has no such object. That case passes there on what it can show.

Checks

• Conformance: 30 of 30 on each of the 7 targets, and nothing is unsupported. The fast compiled forms show no State, so they pass those cases on the output.
• Python tests: 510 pass. Four new tests use runs that really overlap (asyncio.gather). All four fail on the code before this change.
• TypeScript tests pass, with two new tests for overlapping runs.
• Both linters report nothing.
• Speed check (Python 3.12): every number is within 25% of its baseline.

Not in this change

• Fan-out and fan-in (Fan-out and fan-in: work at the same time #12). This change is what Fan-out and fan-in: work at the same time #12 needs first.

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