Reuse observed state for delta commands

[Nzouh]

Summary

• cache joint state from the normal observation cycle
• reuse that state for delta-to-absolute command conversion
• clear the cached state on reset and close
• cover consecutive delta commands and the exact observation read count

This removes the extra full get_observation() call in delta mode, which otherwise reads the serial bus and synchronous cameras twice per step.

Closes #2.

Validation

• python -m pytest --cov (81 passed, 100% coverage)
• python -m ruff check .
• python -m ruff format --check .
• python -m mypy src/inspect_robots_so101/embodiment.py

[Copilot]

Pull request overview

This PR optimizes delta-mode control in SOArmEmbodiment by caching the most recently observed joint state and reusing it to convert delta actions into absolute joint targets, eliminating an extra get_observation() (and associated serial/camera reads) per control step.

Changes:

• Cache the last observed joint state (_last_state) during the normal observation cycle and reuse it for delta-to-absolute command conversion.
• Clear cached state on reset() and close() to avoid stale reuse across episodes/lifecycle boundaries.
• Extend tests to cover consecutive delta commands and assert the exact number of observation reads.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/inspect_robots_so101/embodiment.py	Adds _last_state caching and uses it for delta-mode command conversion; clears cache on reset/close.
tests/test_embodiment.py	Updates the fake driver to count observation reads and validates correct behavior across consecutive delta steps.

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[jeqcho]

Thanks, this is a clean fix. Using the state from the observation the policy actually conditioned on is arguably more correct for delta checkpoints than the old fresh read, since that's what the delta was computed against. The consecutive-delta test pinning the exact observation_reads count is a good regression lock too.

I reproduced your validation locally: 81 passed, 100% branch coverage, ruff and mypy clean, and it merges cleanly with your #18.

One ask before merge: swap the assert self._last_state is not None for an explicit RuntimeError guard mirroring _require_driver() (with the same # pragma: no cover). Asserts vanish under python -O, and the failure would then surface as a confusing None + cmd TypeError.

Also, both of your PRs here are unusually high quality. Claims that reproduce, tests that pin the actual bug, no scope creep. Appreciate it, and I'd be glad to see more.

[jeqcho]

Merged, thanks again. I pushed the RuntimeError guard swap myself so this didn't have to wait on another round trip; everything else is your work as reviewed. This lands in the next release. Both of your PRs were a pleasure to review, and we'd be glad to have more.

[Nzouh]

Thanks Jay, I really appreciate the thoughtful reviews and the merges. I reached out to Aris by email earlier because I'm interested in what Robocurve is building and would love to help more directly if there's room for an intern.