fix(IPC-23,IPC-24,IPC-25,IPC-32): proto-comment regen wave + codegen-freshness guards

Proto comments (comment-only, no wire change):
- mxaccess_worker.proto GatewayHello.max_frame_bytes: every worker->gateway frame
  must serialize within the negotiated max; reply builders truncate (IPC-23).
- mxaccess_gateway.proto DrainEventsReply: count-cap + byte-cap, drain-until-empty
  caller contract (IPC-23).
- mxaccess_gateway.proto ReplayGap.oldest_available_sequence: empty-ring value is
  highest-observed+1, oldest-1 resume formula stays valid (GWC-25 deferred amendment).

Regen wave: Contracts/Generated (C# XML doc), rust vendored protos (byte-copy),
Go bindings (worker binding was genuinely stale - lacked MaxFrameBytes entirely),
Python worker _pb2 (real descriptor delta), Java aggregates (javadoc, zero
protobuf-version churn under the pinned toolchain), client descriptor set.

IPC-24: pinned Java toolchain regenerates with no gencode-version churn, so the
unconditional churn-revert step in ci.yml is a fossil - deleted it; git diff is
now a true message-level drift gate for the single-file Java aggregates.

IPC-25: pin protoc-gen-go v1.36.11 / protoc-gen-go-grpc 1.6.2 in the Go generate
script (+ fix a latent pwsh-7 parse bug); add Check 4 to check-codegen.ps1
(regenerate Go+Python bindings, fail on diff, tool-missing fails not skips); add
the pinned-generator installs to the portable CI job.

IPC-32: relabel check-codegen banners 1/4..4/4 (folded into the Check 4 edit).

Docs: ClientProtoGeneration.md, Contracts.md, GatewayTesting.md, build.gradle
checkGeneratedClean caveat. Tracking: IPC-23/24/25/32 -> Done, GWC-25 proto note
resolved, change-log 2026-08-07.

.gitea/workflows/ci.yml

@@ -60,7 +60,19 @@ jobs:
           dotnet tool install --global PowerShell
           echo "$HOME/.dotnet/tools" >> "$GITHUB_PATH"
 
-      # IPC-01 / IPC-19 / IPC-20: descriptor set + Contracts/Generated must match the current protos.
+      # IPC-25 Check 4 regenerates the Go and Python client bindings and diffs them, so the pinned
+      # generators must be present. protoc 34.1 is already installed above; Go and Python are set up
+      # above. Pin protoc-gen-go / protoc-gen-go-grpc to match the committed header stamps and grpcio
+      # -tools to match the committed _pb2 stamp, or Check 4 false-fails (or masks drift) under churn.
+      - name: Install pinned client codegen generators (Check 4)
+        run: |
+          go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.36.11
+          go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.6.2
+          echo "$(go env GOPATH)/bin" >> "$GITHUB_PATH"
+          python -m pip install 'grpcio-tools==1.80.0'
+
+      # IPC-01 / IPC-19 / IPC-20 / IPC-25: descriptor set + Contracts/Generated + Go/Python bindings
+      # must match the current protos.
       - name: Codegen / descriptor freshness
         shell: pwsh
         run: ./scripts/check-codegen.ps1
@@ -93,10 +105,12 @@ jobs:
           python -m pytest
 
   java:
-    # Java client runs on a JDK-17 Linux runner (the macOS dev box has no JRE). The protobuf gradle
-    # plugin rewrites MxaccessGateway.java with spurious protobuf-runtime-version churn on every
-    # build; when no .proto changed, revert that one file so checkGeneratedClean / a dirty tree does
-    # not fail the build (repo memory project_java_generated_churn).
+    # Java client runs on a JDK-17 Linux runner (the macOS dev box has no JRE). The grpc/protobuf
+    # toolchain is fully pinned (clients/java/build.gradle: grpcVersion 1.76.0 / protobufVersion
+    # 4.33.1), so a regeneration is byte-identical to the committed aggregates modulo real .proto
+    # changes — `Verify generated tree is clean` (git diff) is the true drift gate (IPC-24). The
+    # single-file Java aggregates are where message-level proto drift lands, so this job now catches
+    # a .proto edited without regenerating and committing the Java client.
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -115,13 +129,10 @@ jobs:
       - name: Gradle test
         working-directory: clients/java
         run: gradle test
-      - name: Revert spurious protobuf-version churn (no .proto changed)
-        # Both generated aggregates can pick up protobuf-runtime-version churn on regen; revert
-        # both so verify-clean still catches a real, uncommitted proto/codegen change elsewhere.
-        run: |
-          git checkout -- clients/java/src/main/generated/main/java/mxaccess_gateway/v1/MxaccessGateway.java || true
-          git checkout -- clients/java/src/main/generated/main/java/mxaccess_worker/v1/MxaccessWorker.java || true
       - name: Verify generated tree is clean
+        # IPC-24: the pinned grpc/protobuf toolchain regenerates byte-identical output, so this
+        # git-diff gate now catches message-level proto drift in the single-file Java aggregates
+        # (the old unconditional churn-revert step masked exactly that class and was deleted).
         run: git diff --exit-code -- clients/java/src/main/generated
 
   windows-x86: