feat: add bounded structure-aware Behavior Tree XML diff

[picknik-johnny-5-bot]

[written by AI]

Related to PickNikRobotics/moveit_pro#21717

Summary

• Add BT::RenderXMLDiff as an installed C++17 library API with deterministic Markdown and plain-text output. It distinguishes moved, modified, added, and removed BT content, including wrapper insertions.
• Match exact and similar subtrees with indexed, parent-aware candidates instead of Cartesian candidate lists. Preserve duplicate sibling and reorder semantics, and replace quadratic reorder detection with an O(n log n) prefix-index algorithm.
• Reject malformed UTF-8, embedded NULs, invalid XML code points, and invalid numeric character references before rendering. Emit UTF-8 move markers independent of the Windows execution code page, and avoid duplicate child edits within modified parents.

Validation

• Release build and full CTest suite passed; 31 focused XMLDiff tests passed.
• 31 focused XMLDiff tests passed under ASan/UBSan with leak detection disabled.
• Strict Clang C++17 syntax check passed, as did an installed-package C++17 consumer and an adversarial stress harness.
• Under a 128 MiB address-space limit, 2,000 modified siblings and 3,000 exact duplicate siblings passed. The 200,000-exact-sibling stress case completed in 0.24 seconds after reorder detection was bounded; it timed out at 2 seconds before that fix.
• clang-format 17.0.6 check and git diff --check passed.

Scope and remaining work

• This PR provides the reusable renderer API, not a standalone CLI or CI integration. A consumer can publish its Markdown/plain-text output as a companion review artifact.
• The full sanitizer suite is not green locally: PostConditions.Issue601 intermittently asserts in the timer queue. The same test also intermittently failed in the unchanged base and prior candidate sanitizer builds. Focused sanitizer tests for the changed XML-diff path pass. Leak detection was disabled for the focused run because of a separate pre-existing leak.

CI and review status at aa671ad574505616d1e86d4164a72decce8983ec

• Every current check passed except Pixi Windows. Its unchanged Reactive.MissingOrEmptyGenericChildIdIsRejected test fails with SEH 0xc0000005. The same test failed in the earlier fix/22500-reactive-generic-children run on September 10, before this PR: https://github.com/PickNikRobotics/BehaviorTree.CPP/actions/runs/34393607502. Current failing run: https://github.com/PickNikRobotics/BehaviorTree.CPP/actions/runs/36072102339. This does not establish that the whole Windows job is healthy; a separate baseline fix or explicit exception is needed.
• CodeRabbit's GitHub check skipped this draft. Its local CLI found zero issues at the immediately preceding SHA; the one exact-head attempt failed with a WebSocket closure. No further automated CodeRabbit review was triggered. Request a human rerun or review before marking ready.

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