Extend similar-issue detection across projects, not only within a single project

Problem Statement Similarity detection is scoped to a single project. Cross-Project Issues lets you view issues from several projects in one stream, and Similar Issues will find related issues inside one project, but nothing detects that an issue in project A and an issue in project B are the same defect. Organizations that share code across products hit this constantly: a shared engine, an internal SDK, a common platform or publishing layer. When a defect lands in that shared code it surfaces as separate issues in separate projects, each investigated independently by a different team, often reaching the same conclusion days apart. The inverse case is worse. A shared-layer regression looks like a small local problem in each project, so its true blast radius is never visible to anyone, and prioritisation is wrong by construction: an issue affecting five projects gets triaged as five separate small issues. Project boundaries are organisational. Defects follow code, and shared code c…