Task: Declare the Boost component in CMake, not only the vcpkg port

Close the other half of the same dependency. The vcpkg port makes the header install; the root find_package(Boost ... COMPONENTS ...) is what the estate uses to declare which Boost libraries the project builds against, and what CMake checks before anything compiles.

• The root find_package(Boost ... COMPONENTS ...) lists multiprecision, as it already lists the other header-only libraries the tree uses (crc, scope_exit, circular_buffer).
• A configure with the port declared succeeds and reports the component found.
• A configure with the port undeclared fails, naming boost_multiprecision, before anything compiles.
• The decimal suite still passes and the tree still builds.

1. Add the component to the root list and configure, reading the Found Boost ... found components: line back rather than trusting the exit code alone.
2. Prove the gate by removing the manifest line, reconfiguring, and reading the failure; restore it and reconfigure.
3. Rebuild the affected library and re-run the decimal suite.

Every number this task claims about a tree, with the command that produced it and the commit it was measured at.

• The declaration is what makes the failure early. Without the port, CMake stops at configure time with the component's name; with only the vcpkg line and no component, the same defect appears as a missing header after vcpkg has run and hundreds of files have compiled.
• A component that is header-only still belongs in the list. crc, scope_exit and circular_buffer are all header-only and all listed, and none of them is linked: the estate links Boost::boost for header-only use and specific targets such as Boost::json only where a compiled library is needed. multiprecision joins the list on the same terms, which is why no link line changed.
• The component resolves against vcpkg's Boost because share/boost_multiprecision/boost_multiprecision-config.cmake is installed by the port. It was verified against that tree directly, and separately with a two-file CMake project, because the first attempt picked up the system Boost and reported the component missing – a different Boost, not a different answer.

The root find_package(Boost 1.88 REQUIRED COMPONENTS ...) lists multiprecision, and a configure reports twelve components found, that one among them. The decimal suite passes 71 assertions in 17 cases and ores.utility.tests builds.

The gate is proved in both directions. With the port declared, the configure succeeds. With the manifest line removed, the configure fails before compiling and names boost_multiprecision, which is the failure the original defect should have produced: seconds instead of twenty minutes, and the port to add rather than a header that cannot be found.