Jackson 2 to 3 migration companion code

Three Maven modules - jackson2-before (2.22.1), jackson3-after (3.2.1) and a
coexistence module with BOTH majors on one classpath - so every claim in the two
migration guides is executed rather than asserted. Paired class names make the
before/after outputs directly diffable via run-all.sh.

Confirms the guides on wire-format equivalence (10-case suite, zero mismatches),
classpath coexistence and the collapse of four artifacts into one. Corrects nine
points, including that enableDefaultTyping() is still present in Jackson 2.22.1
rather than removed in 2.16, and that the published "after" mapper snippet does
not compile.

run-all.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+# Builds all three modules and runs every example, writing real output to
+# docs/output/<module>-<Class>.txt. The before/after pairs share a class name so
+# the outputs can be diffed directly:
+#
+#   diff docs/output/before-S07DefaultsThatFlipped.txt \
+#        docs/output/after-S07DefaultsThatFlipped.txt
+set -u
+cd "$(dirname "$0")"
+
+mvn -B -q clean compile || { echo "compile failed"; exit 1; }
+mkdir -p docs/output
+FAILED=0
+
+run_module() {
+  local module="$1" prefix="$2" pkg="$3"
+  local cp
+  cp="$module/target/classes:$(cd "$module" && mvn -B -q dependency:build-classpath \
+        -Dmdep.outputFile=/dev/stdout -DincludeScope=runtime 2>/dev/null | tail -1)"
+  for f in $(find "$module/src/main/java" -name '*.java' | sort); do
+    grep -q 'public static void main' "$f" || continue
+    local short class
+    short=$(basename "$f" .java)
+    class="$pkg.$short"
+    printf '%-14s %-32s' "$prefix" "$short"
+    if java -cp "$cp" "$class" > "docs/output/$prefix-$short.txt" 2>&1; then
+      echo "ok"
+    else
+      echo "FAILED"; FAILED=1
+    fi
+  done
+}
+
+run_module jackson2-before before      com.ankurm.migration.before
+run_module jackson3-after  after       com.ankurm.migration.after
+run_module coexistence     coexistence com.ankurm.migration.coexist
+
+echo
+echo "--- before/after diffs ---"
+for after in docs/output/after-*.txt; do
+  short=$(basename "$after" .txt); short=${short#after-}
+  before="docs/output/before-$short.txt"
+  [ -f "$before" ] || continue
+  if diff -q "$before" "$after" >/dev/null; then
+    echo "  $short: identical output"
+  else
+    echo "  $short: differs (expected — that is the migration surface)"
+  fi
+done
+
+echo
+if [ "$FAILED" -eq 0 ]; then echo "All examples ran successfully."; else echo "Some examples failed."; fi
+exit $FAILED