Remove the Kotlin/JVM implementation; the Rust CLI is now the only ba…

[tinder-maxwellelliott]

…zel-diff

The Rust implementation under src/ has shipped alongside the original Kotlin CLI as //:bazel-diff-rust and passes the same behavioral suite. This makes it the only implementation: //:bazel-diff now resolves to //src:bazel-diff, and everything that existed only to build, test, compare or publish the JVM CLI is gone.

Removed

• cli/ (Kotlin sources, JUnit tests, ktfmt/format targets) and proto/ (the java_proto_library wrappers; the Rust build generates its bindings from rust/proto/*.proto via the cargo build script).
• rules_java, rules_kotlin, rules_jvm_external, the maven extension, stardoc, rules_proto, grpc and the ktfmt repo from MODULE.bazel, plus the rules_kotlin LCOV-merger patch. MODULE.bazel.lock regenerated.
• WORKSPACE support (WORKSPACE, WORKSPACE.bzlmod, repositories.bzl, constants.bzl, artifacts.bzl, extensions.bzl, maven_install.json): the Rust build is crate_universe/bzlmod-only, so nothing could be built through it.
• The Kotlin-vs-Rust comparison tooling (tools/benchmark.py, tools/perf_gate.py, tools/perf_workload.py and their tests, the perf-gate workflow) and docs/kotlin-*.md, which have no second implementation to compare against.
• The Kotlin half of the e2e splitter (tools/e2e), the ktfmt format cron and the JVM CI jobs (test-jre21, test-jre11-run-example's JDK setup, release.yaml's java-tests, the deploy JAR build and release asset).

Moved / repointed

• E2E fixtures: cli/src/test/resources/{fixture,workspaces} ->
  tests/resources/, with .bazelignore, .gitattributes, tests/BUILD and tests/e2e/support/mod.rs following.
• Formatter targets: cli/format -> tools/format (buildifier, rustfmt); //:format is now the rustfmt fixer.
• //release:bazel-diff-rust -> //release:bazel-diff (asset file names keep the bazel-diff-rust-- prefix so existing download URLs still resolve). //:bazel-diff-rust stays as an alias for BCR verify_targets and existing consumers.
• The serve harnesses, the Alpine stress container and the Firecracker bench/guest-image scripts run the static musl binary instead of the deploy JAR; the corresponding workflows build //release:bazel-diff --config=release-musl.
• tools/coverage_check.py defaults to src/; CI's coverage matrix is now the coverage job and gates Rust and Go independently.
• README regenerated from the template with the Rust CLI's help text; installation now covers prebuilt binaries and bzlmod only.

Verified: bazel build //:bazel-diff (Bazel 8.7.0) with the trimmed module graph, cargo test --lib --bin, and the Python tooling tests.