[tsaubergine]

This PR is stacked on #2 and targets its branch, so the diff shows only this change. Merge #2 first; this PR then retargets to main.

This is Phase 0 of the Goby helm plan: what the helm library and the Goby app need before any of their code.

Changes

• init.sh adds the packages.gobysoft.org release repository (key 19478082E2F8D3FE, in /usr/share/keyrings/gobysoft-archive-keyring.gpg) and installs libgoby3-dev and goby3-apps. The repository has packages for all four CI configurations: Ubuntu 24.04 and 26.04, amd64 and arm64.
• CMake
  • looks for Goby3 with find_package(GOBY 3.0 COMPONENTS zeromq), and adds src/bin/goby only if it is found, so the helm library still builds without Goby;
  • builds libraries shared by default (BUILD_SHARED_LIBS), as openocean-messages does. That way goby zeromq publish -l libivpico_interface.so can load the interface messages, and there's no need for a separate poke app.
• external/moos-ivp moves to Make HelmEngine, ContactLedger and contact alerts usable without MOOS moos-ivp#2 (stacked on Build the helm libraries without MOOS, for a helm outside MOOS moos-ivp#1). It adds HelmEngine, ContactLedger and the contact-manager alerts to the helm libraries. Before merging: once both moos-ivp PRs have merged, move the pointer to the merge commit on GobySoft's main.
• README: the Building section mentions Goby3.

.clang-format was already goby3's, so it needed no change.

Test plan

• In clean ubuntu:24.04 and ubuntu:26.04 containers, from a fresh clone of this branch: ./init.sh installs Goby3 3.6.1 from packages.gobysoft.org, ./build.sh finds Goby (goby, goby_zeromq), ctest passes, and git status (submodules included) is clean.
• Without Goby3, CMake prints "Goby3 not found: not building the Goby app" and the build and tests still pass.
• CI on all four jobs.

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