fix: make runtime cleanup and transitions consistent

[jaagut]

Makes runtime transitions iterative and ensures discarded frames are cleaned exactly once, including interrupts and behavior reloads. Callbacks that mutate the stack cancel stale traversal; cleanup runs before a chain successor is initialized, and cleanup errors do not skip the remaining detached frames. Behavior parse/binding failures leave the active behavior intact, and activation parameters are independent copies.

Retains existing action-chain timing, root-pop behavior, suppression scope, and ordinary log-and-continue errors. Exposes the first tick error as last_error and replaces library sys.exit with ValueError for invalid decision outcomes. Debug publishing runs after execution, cannot prevent actions from running, and publishes the current-action state to late subscribers. Discovery restores sys.path on failure and fixes init.py discovery. The execution contract is documented in docs/runtime.md; local comments explain callback mutation guards, cleanup ordering, and chain suppression.

Validation: 161 tests and strict core/example typing pass, including deep decision traversal, cleanup/reload/mutation regressions, nested/cyclic debug data, and a real localhost DDS test for all debug topics and late subscribers. Pre-commit passes. No robot/hardware execution was performed.

Open choices are tracked in #115 (failure/outcome policy), #116 (integration/registration boundaries), and #121 (root chains/suppression); this PR preserves the relevant compatibility behavior.

Stack: based on #120 → #119 → #118 → #114. Visualization regression tests and documentation polish follow in #123.

I am working on behalf of @jaagut.

Full stack, bottom to top: #114 → #118 → #119 → #120 → #122 → #123. Merge in that order.