`bazelisk build //:openroad` previously broke unless the nested git
submodule `third-party/slang-elab/third_party/fmt` was initialized.
That fmt is not an OpenROAD fork — it is plain fmtlib/fmt pulled in
unmodified via a nested submodule. The build failed with an opaque
`glob pattern 'include/fmt/*.h' didn't match anything` when the
nested submodule was missing.

Switch Bazel to BCR (`bazel_dep(name = "fmt", version = "11.2.0.bcr.1")`).
The existing `@fmt` references in `third-party/slang-elab/src/BUILD`
and `src/syn/src/elab/BUILD` resolve to the BCR module instead of the
nested submodule. The slang side stays as `new_local_repository` for
now (slang-elab pins a commit newer than slang v10.0, the latest BCR
release; that switch is a separate follow-up).

CMake builds are unaffected — `.gitmodules` still references the
nested fmt submodule. Only Bazel's view of `@fmt` changes.

Document the submodule-for-CMake / BCR-for-Bazel policy in README.md
so future contributors do not add submodules just to feed Bazel.

Test plan: verified `bazelisk build //:openroad` succeeds with
`third-party/slang-elab/third_party/fmt` deinit'd
(`git submodule deinit -f third_party/fmt`).
`bazelisk mod show_repo fmt` confirms the repo is sourced from
bcr.bazel.build/modules/fmt/11.2.0.bcr.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>