feat: Add Finally control node for guaranteed cleanup

[dv-picknik]

[written by AI]

Adds a Finally control node with two children, main and cleanup. It ticks main, then always ticks cleanup, which gives Objectives try/finally semantics for resetting scene state such as collision rules.

<Finally>
  <Sequence>
    <SetCollisionRule .../>
    <ComplexOperation/>
  </Sequence>
  <SetCollisionRule .../>
</Finally>

Cleanup runs in each of these cases:

• main returns SUCCESS, FAILURE, or SKIPPED. The node returns main's status.
• main throws. The node prints what() to stderr, halts main, runs cleanup, and returns FAILURE.
• the node is halted while main is RUNNING. It halts main and ticks cleanup once, synchronously. If cleanup returns RUNNING, it gets halted.

If cleanup fails, the node returns FAILURE even when main succeeded, because a failed cleanup can leave the scene dirty. An exception thrown by cleanup propagates from tick(). During halt() it is printed instead, because ~Tree() calls haltTree() and a throw there would call std::terminate. The XML loader rejects a Finally without exactly 2 children.

Related: PickNikRobotics/moveit_pro#20169, milestone 10.2.0.

TryCatch, which arrived with the 4.9.0 port, doesn't cover this. It skips the catch child on SUCCESS and does not catch exceptions.

Open for review:

• Converting a main exception to FAILURE follows the issue text, but it drops the NodeExecutionError backtrace everywhere except stderr. The other option is to store a std::exception_ptr and rethrow once cleanup finishes. That keeps the diagnostics, but an outer Fallback could no longer recover.
• I skipped the single-child decorator variant the issue mentions. It can come later if someone needs it.
• Finally exists only in our fork. It should go upstream as part of the BT.CPP unfork work.

Validation: behaviortree_cpp_picknik_test passes all 542 tests locally (Release, GCC). That includes 17 new FinallyTest cases covering async main and cleanup, a halt in each phase, exceptions from main and from cleanup on both the tick and halt paths, SKIPPED main, and child-count validation. pre-commit passed on the changed files. picknik:code-reviewer and the CodeRabbit CLI both flagged cleanup throwing during halt, and the second commit fixes it. Writing that SKIPPED test turned up a double cleanup: executeTick() leaves the node RUNNING after a SKIPPED return, so a later halt ran cleanup again. The node now resets itself before returning SKIPPED.

🤖 Generated with Claude Code

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