Abort safely when cc_rbptree node allocation fails

[Dikluwe]

Problem

The third and subsequent rbptree insertions allocate nodes. A failed allocation was dereferenced before the API could report anything; insert() has no failure return.

Changes

• Check node allocation and use Coin's existing explicit OOM abort policy from Handle OOM in cc_dict consumers and supporting primitives #772.
• Add a positive insertion control and a deterministic failure of the first allocated node, expecting SIGABRT instead of invalid memory access.

Tree representation, public signatures and borrowed pointer/data ownership are unchanged. This contribution is the rbptree node check and its regression; the allocator and dictionary changes belong to #772.

Dependencies — keep as draft

Depends on #772. 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: c434d067dc8686893ee3dcb9e38123766e365904 (codex/cc-oom-consumers). Review this contribution over its base.

Validation

Linux candidate 98/98 CTests at 3b5e04b1e98164157405623e43d0d6da61343644, with no skips. Dedicated integration lab previously passed 140/140 at bae0edd15cd859aee8165d20a37026661854bfba; 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.