Task: Implement Hotfix: a Boost library landed without its vcpkg port

Declare the port the decimal type needs, prove the translation unit and its consumer build against a vcpkg tree that did not have it, and leave a check that names the port the next time a Boost library arrives without one.

• A clean configure installs boost-multiprecision, and the decimal translation unit compiles against it.
• The decimal suite and the bond leg amount suite pass.
• check_boost_dependencies.py passes on the tree, and fails when the port is removed from the manifest, naming the include and the line to add.
• The check runs before the Continuous Linux build.

1. Add the port to vcpkg.json and reconfigure, so vcpkg itself proves the header is installed rather than assumed.
2. Build the decimal unit and a consumer, and run their suites.
3. Write the check against the vendored ports' own manifests, and prove it both ways: passing on the tree, failing with the port removed.
4. Wire it into the Linux workflow before the build, then raise the pull request.

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

• The header was genuinely absent from this worktree's vcpkg tree as well, which is why the failure is not a CI cache artefact: the tree had never installed the port, and the include compiled nowhere. That is also why the check could be proved cheaply, by removing the line and re-running it.
• The check accepts a library that arrives transitively. boost-math is installed because boost-multiprecision depends on it and is not declared; a check that demanded one line per include would fail the tree for being right.
• The manifest turned out not to be the only file the build needed. The root find_package(Boost ... COMPONENTS ...) declares every Boost library the project builds against, header-only ones included, and the follow-up task Declare the Boost component in CMake, not only the vcpkg port adds multiprecision there. That is the declaration that fails at configure time when the port is missing, which this task's own run confirmed only after the fact. No link line changes: the header-only libraries already in the list are not linked either.
• A pull-request-time version of the check would need the workflow to fetch the vcpkg submodule, which no pull-request workflow does today. The story records that option rather than taking it, because it changes the cost of every pull request.

vcpkg.json declares boost-multiprecision, and a clean configure of this worktree's preset installed boost-multiprecision:x64-linux@1.91.0 (with boost-math, which it depends on). The decimal translation unit compiles, the decimal suite passes 71 assertions in 17 cases, and the bond leg amount suite, which consumes the type the commit converted, passes 28 assertions in 3 cases.

build/scripts/check_boost_dependencies.py compares the boost/<library>/ directories the tree includes with the closure of the declared ports, read from the ports' own manifests. On this tree it reports 18 directories used and 116 ports installed, with nothing missing; with the new line removed from the manifest it fails and names the include and the line to add. Continuous Linux runs it before the build.