Give SoCallbackList copy and assignment a strong exception guarantee

[Dikluwe]

Problem

Implicit copy/assignment and ownership-registry updates could partially change a destination on allocation failure. Cleanup also copied registry entries and could allocate while destroying callbacks.

Changes

• Stage copy/assignment completely before committing pointer storage and ownership state.
• Add explicit out-of-line SoCallbackList copy constructor and assignment operator; handle self-assignment and empty/mixed owned and raw callbacks.
• Inspect existing SbHash values by pointer so removal/cleanup can release records without allocation.
• Add narrow internal SbPList friendship without changing its layout, and deterministic bad_alloc copy/assignment/lifetime oracles.

The public class layout remains unchanged, but new copy methods add symbols. Recompiled clients use the strong guarantee; clients retaining the old inline/implicit implementation do not acquire that guarantee automatically. Old/new-header client link/run and sizeof/alignof checks passed in the reviewed combined library; formal ABI and native-platform checks remain CI gates. The fault oracle runs on Linux shared builds and concerns throwing C++ allocations, not generalized recoverable OOM.

Dependencies — keep as draft

Depends on #831. The diff against master includes the prerequisite stack. The contribution over the reviewed base is linked below; narrow/rebase the PR after its prerequisites are integrated.

Base: ffba47d0b5ff664790ab6277b39dc71c6f07ab65 (codex/pr/socallbacklist-add-atomic). Review this contribution over its base.

Validation

Linux candidate 87/87 CTests at ee516a61ecd17ae79178d9cf5d007768ebb3f242, with no skips. Dedicated integration lab previously passed 141/141 at f987241ef1b686e287fa43a2c143d5d7644e4988; that lab count is a separate reviewed snapshot.

Fresh publication gate of all eight exact heads: 35/35 directed and 153/153 total CTests, no skips, at e6687c088077082e6839b4fc39c632d5267b07d8. GCC/Ninja Debug shared build with COIN_THREADSAFE=ON and external Expat, build and compiler temporary files in RAM; Xvfb/software GLX environment. git diff --check passed for the proposed delta. Native Windows/macOS and formal ABI checks rerun on this published head.