freemkv-unlock had no automated checks at all: no lint, no tests, no
build on any OS but the one a developer happened to be sitting at. It is
the base of the dependency graph — libfreemkv path-deps it, bdemu
path-deps it with the emulation feature, and the rest pull it in
transitively — so a break here takes all seven other repos down, and
until now the first machine to notice would have been the release
workflow.
Full OS matrix, matching libfreemkv's: lint and test on Linux, plus
macOS and Windows building the tests (not `cargo check`, which skips
codegen and never compiles test-only code).
The `emulation` feature gets its own test run. It is off by default and
it is the face bdemu builds against, so nothing was compiling it.
Also adds the leak-guard workflow the other public repos already carry.
