Adds `cargo llvm-cov` to the devshell and a coverage workflow that runs
only when someone asks for it.
Manual dispatch rather than nightly or per-PR, per the discussion on the
issue: a coverage run needs its own instrumented build and cannot reuse
the normal test artifacts, so it roughly doubles a test job. "Do we have
glaring holes" is a question someone asks occasionally, not a gate every
PR pays for, and not a number worth spending farm time on every night
whether or not anyone reads it. Manual dispatch costs nothing until the
answer is wanted.
Its own workflow file rather than a job in ci.yml: that file already has
a `workflow_dispatch` trigger so `hive-forge ci-rerun` can retrigger
without an empty commit, and a trigger there fires EVERY job in the
file -- a coverage job added there would run on every pull request.
No threshold and no --fail-under-lines. A coverage gate mostly teaches
people to write assertion-free tests that execute lines; the report is
the deliverable and the number is for a human to read.
The devshell needs LLVM_COV / LLVM_PROFDATA set explicitly: cargo
llvm-cov expects rustup's `llvm-tools-preview` beside the toolchain and
nixpkgs has no such component, so without them it aborts with "failed to
find llvm-tools-preview" -- which reads like a missing install rather
than a path the shell has to name.
Verified by running it: `cargo llvm-cov --package hive-types` produces a
real report (3 tests, 85.44% regions). Both the variable names and the
package were wrong on the first attempt and only running it said so --
the names take no `_PATH` suffix, and the binaries are in
`llvmPackages.llvm`, not `llvmPackages.bintools`, which is the linker
wrapper and ships neither.
