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/docs/output/04-two-executioncontext-classes.txt

@@ -0,0 +1,48 @@
+# javap: two ExecutionContext classes in two different packages
+
+$ javap -cp spring-batch-infrastructure-6.0.5.jar org.springframework.batch.infrastructure.item.ExecutionContext
+Compiled from "ExecutionContext.java"
+public class org.springframework.batch.infrastructure.item.ExecutionContext implements java.io.Serializable {
+  public org.springframework.batch.infrastructure.item.ExecutionContext();
+  public org.springframework.batch.infrastructure.item.ExecutionContext(java.util.Map<java.lang.String, java.lang.Object>);
+  public org.springframework.batch.infrastructure.item.ExecutionContext(org.springframework.batch.infrastructure.item.ExecutionContext);
+  public void putString(java.lang.String, java.lang.String);
+  public void putLong(java.lang.String, long);
+  public void putInt(java.lang.String, int);
+  public void putDouble(java.lang.String, double);
+  public void put(java.lang.String, java.lang.Object);
+  public boolean isDirty();
+  public java.lang.String getString(java.lang.String);
+  public java.lang.String getString(java.lang.String, java.lang.String);
+  public long getLong(java.lang.String);
+  public long getLong(java.lang.String, long);
+  public int getInt(java.lang.String);
+  public int getInt(java.lang.String, int);
+  public double getDouble(java.lang.String);
+  public double getDouble(java.lang.String, double);
+  public java.lang.Object get(java.lang.String);
+  public <V> V get(java.lang.String, java.lang.Class<V>);
+  public <V> V get(java.lang.String, java.lang.Class<V>, V);
+  public boolean isEmpty();
+  public void clearDirtyFlag();
+  public java.util.Set<java.util.Map$Entry<java.lang.String, java.lang.Object>> entrySet();
+  public java.util.Map<java.lang.String, java.lang.Object> toMap();
+  public boolean containsKey(java.lang.String);
+  public java.lang.Object remove(java.lang.String);
+  public boolean containsValue(java.lang.Object);
+  public boolean equals(java.lang.Object);
+  public int hashCode();
+  public java.lang.String toString();
+  public int size();
+}
+
+$ javap -cp spring-batch-core-6.0.5.jar org.springframework.batch.core.repository.persistence.ExecutionContext
+Compiled from "ExecutionContext.java"
+public final class org.springframework.batch.core.repository.persistence.ExecutionContext extends java.lang.Record {
+  public org.springframework.batch.core.repository.persistence.ExecutionContext(java.util.Map<java.lang.String, java.lang.Object>, boolean);
+  public final java.lang.String toString();
+  public final int hashCode();
+  public final boolean equals(java.lang.Object);
+  public java.util.Map<java.lang.String, java.lang.Object> map();
+  public boolean dirty();
+}