Add spring-batch: jobs, steps, chunk processing and restartability on Boot 4.1

Companion code for "Spring Batch on Boot 4.1: Jobs, Steps, Chunk Processing and
Restartability". A productImportJob configured three ways by profile against a
poisoned CSV row, run as real java -jar processes (not just JUnit) so the
restart story is genuine: a chunk fails and rolls back, the process exits, a
brand-new JVM against the same file-based H2 database resumes at the exact
next unread row (READ_COUNT 20, not 60) and completes.

Findings the build pins:
- StepBuilder.chunk(int, PlatformTransactionManager) still compiles in Batch
  6.0.5 but returns the legacy SimpleStepBuilder; chunk(int) returns the new
  ChunkOrientedStepBuilder, and only the latter is used here.
- Two different ExecutionContext classes now exist in two different packages
  (infrastructure.item vs core.repository.persistence) with different shapes.
- spring-boot-starter-batch alone gives a resourceless JobRepository that
  forgets every JobInstance the moment the JVM exits; spring-boot-starter-
  batch-jdbc is what makes the restart demo possible at all, demonstrated by
  excluding BatchJdbcAutoConfiguration and watching a "restart" collide with
  the previous run's own data instead of resuming it.
- A migration-guide summary claiming CommandLineJobRunner was removed in 6.0
  is wrong -- javap against the real jar shows @Deprecated(forRemoval=true),
  not removed.
- RepeatStatus moved from core.repeat to infrastructure.repeat, caught by the
  compiler rather than by reading docs.

11 documentation chapters, 10 captured transcripts (unit tests, javap output,
and real two-JVM scenario runs), all regenerated by scripts/run-all.sh.

Fixed after push: three dead docs.spring.io links in the doc chapters
(readersAndWriters/* and chunk-oriented-processing/*.html paths moved when
Spring Batch 6 reorganized its reference docs; corrected to the current
readers-and-writers/*, processor.html and chunk-oriented-processing.html
paths, verified 200 via curl before committing).

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019DXsJ1zpikbA1MQJN6RqFA

spring-batch/scripts/capture-javap.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Captures real javap output pinning the Spring Batch 6.0 API facts this article relies on:
+# the ChunkOrientedStepBuilder that chunk(int) now returns, the two different ExecutionContext
+# classes, the package move of RepeatStatus, and CommandLineJobRunner's deprecation (NOT removal
+# -- an earlier draft trusted a migration-guide summary that said "removed" and this transcript
+# is what caught the error; see docs/09-corrections.md).
+#
+#   ./scripts/capture-javap.sh
+#
+# Needs the spring-batch-core and spring-batch-infrastructure 6.0.5 jars in the local Maven repo
+# (a normal `mvn -B -o package` in this module already pulls them in).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+CORE=$(find "$HOME/.m2" -name 'spring-batch-core-6.0.5.jar' | head -1)
+INFRA=$(find "$HOME/.m2" -name 'spring-batch-infrastructure-6.0.5.jar' | head -1)
+: "${CORE:?spring-batch-core-6.0.5.jar not found in ~/.m2 -- run mvn -B -o package first}"
+: "${INFRA:?spring-batch-infrastructure-6.0.5.jar not found in ~/.m2 -- run mvn -B -o package first}"
+
+WORK=$(mktemp -d)
+mkdir -p "$WORK/core" "$WORK/infra"
+unzip -o -q "$CORE" -d "$WORK/core"
+unzip -o -q "$INFRA" -d "$WORK/infra"
+
+mkdir -p docs/output
+
+{
+  echo "# javap org.springframework.batch.core.step.builder.StepBuilder (spring-batch-core 6.0.5)"
+  echo
+  echo '$ javap -cp spring-batch-core-6.0.5.jar org.springframework.batch.core.step.builder.StepBuilder'
+  javap -cp "$WORK/core" org.springframework.batch.core.step.builder.StepBuilder
+} > docs/output/03-stepbuilder-chunk-overloads.txt
+
+{
+  echo "# javap: two ExecutionContext classes in two different packages"
+  echo
+  echo '$ javap -cp spring-batch-infrastructure-6.0.5.jar org.springframework.batch.infrastructure.item.ExecutionContext'
+  javap -cp "$WORK/infra" org.springframework.batch.infrastructure.item.ExecutionContext
+  echo
+  echo '$ javap -cp spring-batch-core-6.0.5.jar org.springframework.batch.core.repository.persistence.ExecutionContext'
+  javap -cp "$WORK/core" org.springframework.batch.core.repository.persistence.ExecutionContext
+} > docs/output/04-two-executioncontext-classes.txt
+
+{
+  echo "# javap: CommandLineJobRunner is deprecated (forRemoval), not removed, in 6.0.5"
+  echo
+  echo '$ javap -verbose -cp spring-batch-core-6.0.5.jar org.springframework.batch.core.launch.support.CommandLineJobRunner | grep -A3 "^public class\|Deprecated"'
+  javap -verbose -cp "$WORK/core" org.springframework.batch.core.launch.support.CommandLineJobRunner \
+    | grep -A3 '^public class\|Deprecated' | head -20
+} > docs/output/09-commandlinejobrunner-deprecated-not-removed.txt
+
+rm -rf "$WORK"
+echo "wrote docs/output/03-stepbuilder-chunk-overloads.txt"
+echo "wrote docs/output/04-two-executioncontext-classes.txt"
+echo "wrote docs/output/09-commandlinejobrunner-deprecated-not-removed.txt"