CVE-2026-91777: jackson-databind quadratic forward-reference completion

### Summary When an `@JsonIdentityInfo` collection or map first creates N unresolved object-ID references and later resolves the same IDs in reverse order, jackson-databind scans the remaining pending-reference accumulator for each resolution. A shallow JSON document whose size grows linearly can therefore cause quadratic CPU work during deserialization. ### Details The affected path is forward-reference completion in `CollectionDeserializer.CollectionReferringAccumulator.resolveForwardReference()` and the corresponding map implementation. The implementation performs a linear search of the pending accumulator for every resolved object ID. The behavior is runtime-confirmed in jackson-databind 2.5.0, 2.22.1, and 3.2.1. Current 2.22 and 3.2 source branches retained the same design when rechecked. A 2.4.0 control fails closed before successful reverse-order completion, so 2.5.0 is the conservative runtime-confirmed affected floor. The patched versions are: 2.18.11, 2.21.7, 2.22.3, 3.1.7 and 3.2.3. The vulnerable application must deserialize attacker-influenced JSON into an identity-enabled collection or map. The issue does not require deep nesting or syntactically unusual JSON. Suggested correction: replace repeated linear lookup/removal with a keyed pending-reference structure or another design that provides linear or amortized-linear completion. A regression should preserve input order, duplicate-ID behavior, and unresolved-ID errors while bounding reverse-order resolution work. ### PoC The proof constructs a shallow collection containing N unresolved `@JsonIdentityInfo` references followed by definitions of those same IDs in reverse order. Its ID class counts `equals()` calls, giving a deterministic work measure rather than a timing-dependent result. With N=2,000, affected versions perform exactly 2,003,000 ID comparisons. An equally sized control in which every reference is already resolved performs zero comparisons in the pending-reference lookup path. The run is bounded to a 512 MiB JVM. The result demonstrates quadratic growth: approximately `N * (N + 1) / 2` comparisons, plus fixed setup comparisons. ### Impact An unauthenticated source that can submit JSON to an application using the affected identity-enabled collection or map shape can consume quadratic CPU and exhaust a request-time or worker-capacity budget, causing denial of service. The application model/configuration prerequisite is material. No confidentiality, integrity, code-execution, or parser-depth impact is claimed. Requested credit: Daniel Birtwhistle