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/run-scenario.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Runs one scenario against its own external H2 file database and CSV drop file (both under
+# scenario-data/<name>/, outside target/ so a rebuild never touches them -- see
+# docs/07-restartability.md for why that matters: `mvn spring-boot:run` re-copies
+# src/main/resources over target/classes before every run, which would silently undo an
+# in-place "fix" to a classpath resource between two runs of the same scenario.
+#
+#   ./scripts/run-scenario.sh <name> <profile> <source-csv>
+#
+# Reuses the packaged jar (target/spring-batch-1.0.0.jar); run `mvn -B -o -DskipTests package`
+# first if it is missing or stale.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+NAME="${1:?usage: run-scenario.sh <name> <profile> <source-csv>}"
+PROFILE="${2:?usage: run-scenario.sh <name> <profile> <source-csv>}"
+SRC_CSV="${3:?usage: run-scenario.sh <name> <profile> <source-csv>}"
+
+DIR="scenario-data/${NAME}"
+mkdir -p "$DIR"
+if [ ! -f "$DIR/input.csv" ]; then
+  cp "$SRC_CSV" "$DIR/input.csv"
+fi
+
+java -jar target/spring-batch-1.0.0.jar \
+  --spring.profiles.active="$PROFILE" \
+  --import.file="file:${DIR}/input.csv" \
+  --spring.datasource.url="jdbc:h2:file:${PWD}/${DIR}/db"