Document how to regenerate the binding by hand

GENERATION.md

@@ -34,3 +34,43 @@ Tracking master keeps the source and the `@master` derivation toolchain moving t
 so the catalog and jar never drift from the `run.py` that derives them.
 
 The UDF layer is the generated `registerAll()` — zero hand-written registrations.
+
+## Regenerating by hand
+
+CI performs the steps below via `provision-meos`. To run them yourself you need a JDK, Maven,
+CMake and the MEOS build dependencies.
+
+**1. Derive libmeos and the catalog from MobilityDB master.** Both come from one commit; see
+`MEOS-API/GENERATION.md` for the two commands:
+
+```bash
+MDB=~/src/MobilityDB                     # checkout at the commit you are deriving from
+MEOSAPI=~/src/MEOS-API
+cmake -S "$MDB" -B "$MDB/build" -DCMAKE_BUILD_TYPE=Release -DMEOS=ON -DALL=ON
+cmake --build "$MDB/build" -j"$(nproc)"
+cmake --install "$MDB/build" --prefix "$MDB/.prefix"
+cd "$MEOSAPI" && MDB_SRC_ROOT="$MDB" python3 run.py "$MDB/.prefix/include"
+```
+
+**2. Build the JMEOS jar against that catalog and install it into the local Maven
+repository** under the coordinates this build resolves — `org.jmeos:meos:1.0`:
+
+```bash
+cd ~/src/JMEOS                            # JMEOS main
+CATALOG="$MEOSAPI/output/meos-idl.json" LIBMEOS="$MDB/.prefix/lib/libmeos.so" \
+  tools/regen-from-catalog.sh
+mvn install:install-file -Dfile=jar/JMEOS.jar \
+  -DgroupId=org.jmeos -DartifactId=meos -Dversion=1.0 -Dpackaging=jar
+```
+
+**3. Stage the catalog and build.** `tools/meos-idl.json` is derived, not committed:
+
+```bash
+cd ~/src/MobilitySpark
+cp "$MEOSAPI/output/meos-idl.json" tools/meos-idl.json
+LD_LIBRARY_PATH="$MDB/.prefix/lib" mvn clean test
+```
+
+`generate-sources` runs `tools/codegen_jvm.py --engine spark --catalog tools/meos-idl.json
+--jar <the installed jar> --out target/generated-sources/spark/...`, so the UDF layer is
+regenerated by the build itself. `LD_LIBRARY_PATH` is how the tests find `libmeos.so`.