fix(rollout): persist post-action frames

[Vedangalle]

Summary

Persist every distinct rollout observation through FrameStore, including the terminal post-action state.

Previously, the rollout stored only each step's pre-action observation. That left full camera arrays inside StepResult.observation and gave the terminal observation no sidecar reference, because it never becomes a later step's pre-action input.

Changes

• store the reset observation at index 0 and each step result at t + 1;
• add StepRecord.result_image_refs for post-action frame handles;
• strip camera arrays from both recorded pre-action and post-action observations;
• reuse each stored post-action view on the next step, preventing duplicate writes;
• document the storage and compatibility contract in plans/0032-post-action-frame-persistence.md;
• add regression coverage for contiguous indexing, exact write count, and terminal-frame recovery.

A one-camera rollout with n steps now writes exactly n + 1 frame files.

Compatibility

• result_image_refs is appended with a default of None.
• Runs without FrameStore preserve the original observation and result objects.
• StepRecord is not part of the schema-versioned EvalLog, so no log migration is required.
• Core remains NumPy-only and no exported symbol changes are required.

Verification

• Tests added/updated, including terminal-state and exact-write-count coverage
• Full local suite on current main: 876 passed, 3 skipped
• Coverage: 100% line and branch
• ruff check . passes
• ruff format --check . passes
• Strict mypy passes
• CHANGELOG.md updated under "Unreleased"
• Core remains NumPy-only

The gap surfaced while building a process-aware evaluator on top of Inspect Robots: https://github.com/Vedangalle/inspect-robots-verifier

[jeqcho]

Heads up: main's history was rewritten today to purge 3 GB of accidentally committed eval logs (#212). Your branch was based on one of the rewritten commits, so I rebased it onto the new main and force-pushed (maintainer edits are enabled on this PR). No changes to your commits themselves; CI will re-run. If you have local work on this branch, run git fetch and rebase onto the updated remote branch before pushing.

[Vedangalle]

@jeqcho When you have a chance, I’d appreciate your (any) feedback on the FrameStore contract here - particularly whether result_image_refs is the right interface for post-action and terminal observations. Happy to adjust quickly! :)