Polish JVM bindings per code review feedback

Addresses five concerns about the initial Java/Kotlin bindings:

1. Use a conventional reverse-DNS Java package. The bare 'package scip;'
   collided with the outer class name 'Scip' (FQN 'scip.Scip') and
   confused tools like Bazel java_proto_library strict-deps, ProGuard/R8,
   and OSGi. Set java_package to 'org.scip_code.scip' (matching the
   verified scip-code.org domain), so messages live at FQNs like
   'org.scip_code.scip.Index'.

2. Set java_multiple_files = true. Previously the entire ~23k-line
   generated output sat inside one outer 'Scip' class, forcing
   nested-class call sites like Scip.Index.newBuilder(). With
   multiple-files, each message becomes its own top-level class:
   'import org.scip_code.scip.Index'. Picked now because this is a
   wire-compatible source-API change but very expensive to flip
   post-1.0. Also set java_outer_classname = 'ScipProto' so the
   file-level descriptor holder doesn't collide with any 'Scip*'
   message names that the new layout exposes.

3. Declare protobuf-java-util as a dependency in both poms. JsonFormat
   and the related util classes are something most real-world consumers
   eventually reach for; declaring it transitively saves them from
   discovering the missing dep at runtime via NoClassDefFoundError.

4. Ship the canonical scip.proto schema as a Maven classifier artifact
   in both bindings (e.g. 'mvn dependency:get
   org.scip-code:scip-java:0.7.1:proto:proto'), so Bazel/Buck/Pants
   users and consumers generating bindings in another language can pull
   the schema directly instead of vendoring it. Implemented via
   build-helper-maven-plugin's attach-artifact goal, with
   proto-generate copying scip.proto into bindings/{java,kotlin}/src/
   main/proto/scip.proto so the Maven build can find it. The copies
   are excluded from buf's input set in buf.yaml to avoid duplicate
   top-level proto symbols.

5. Document the protobuf 4 gencode-vs-runtime compatibility window in
   per-binding READMEs (new bindings/{java,kotlin}/README.md). Pinning
   protobuf-java to a strict version was rejected because it would
   break Maven/Gradle dependency convergence with consumers using a
   newer 4.x; using a version range was rejected because it would
   break Nix mvnHash determinism. Documenting the upgrade direction
   is the established protobuf-project recommendation.

Side effects:

  * The haskell and rust bindings get regenerated to reflect the new
    option ordering in scip.proto (alphabetized to satisfy buf format).
    Only embedded file descriptors / line numbers shift; no API
    changes.
  * .gitattributes patterns updated for the new java_multiple_files
    layout under org/scip_code/scip/.
  * checks.nix mvnHash refreshed to account for the new
    protobuf-java-util and build-helper-maven-plugin deps.

Author: jupblb

.gitattributes

@@ -2,8 +2,8 @@ bindings/typescript/scip.ts linguist-generated=true
 bindings/go/scip/scip.pb.go linguist-generated=true
 bindings/rust/src/generated/scip.rs linguist-generated=true
 bindings/haskell/src/Proto/**.hs linguist-generated=true
-bindings/java/src/main/java/scip/Scip.java linguist-generated=true
-bindings/kotlin/src/main/java/scip/Scip.java linguist-generated=true
-bindings/kotlin/src/main/kotlin/scip/**.kt linguist-generated=true
+bindings/java/src/main/java/org/scip_code/scip/**.java linguist-generated=true
+bindings/kotlin/src/main/java/org/scip_code/scip/**.java linguist-generated=true
+bindings/kotlin/src/main/kotlin/org/scip_code/scip/**.kt linguist-generated=true
 docs/scip.md linguist-generated=true
 yarn.lock linguist-generated=true