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/05-happy-path.txt

@@ -0,0 +1,19 @@
+# The happy path: 60 rows in, 58 products out
+
+$ java -jar target/spring-batch-1.0.0.jar --spring.profiles.active=clean \
+    --import.file=file:scenario-data/clean-demo/input.csv \
+    --spring.datasource.url=jdbc:h2:file:.../scenario-data/clean-demo/db
+
+2026-09-13T06:23:05.509Z  INFO 5242 --- [           main] o.s.batch.core.step.AbstractStep         : Executing step: [importStep]
+2026-09-13T06:23:05.530Z  WARN 5242 --- [           main] c.a.b.p.ProductValidatingProcessor       : filtering abc-0012: does not match ^[A-Z]{3}-\d{4}$
+2026-09-13T06:23:05.543Z  WARN 5242 --- [           main] c.a.b.p.ProductValidatingProcessor       : filtering ABC-0033: priceCents must be positive, was 0
+2026-09-13T06:23:05.570Z  INFO 5242 --- [           main] o.s.batch.core.step.AbstractStep         : Step: [importStep] executed in 61ms
+2026-09-13T06:23:05.590Z  INFO 5242 --- [           main] o.s.batch.core.step.AbstractStep         : Executing step: [reportStep]
+REPORT: 58 products now in the PRODUCT table
+2026-09-13T06:23:05.600Z  INFO 5242 --- [           main] o.s.batch.core.step.AbstractStep         : Step: [reportStep] executed in 13ms
+JOB FINISHED: status=COMPLETED exitCode=COMPLETED
+
+$ SELECT read_count, filter_count, write_count, commit_count FROM BATCH_STEP_EXECUTION;
+READ_COUNT | FILTER_COUNT | WRITE_COUNT | COMMIT_COUNT
+60         | 2            | 58          | 6
+(1 row, 21 ms)