Add every example from the post, plus edge cases, to the companion repo

Full companion repo for the ankurm.com post "Spring Security Context Propagation:
The Complete Guide" -- every code example the post discusses now has a corresponding
runnable, verified demo (JDK 25, Spring Security 7.1.1, Spring Boot 4.1.1 dependency
versions), not just the virtual-thread/structured-concurrency sections:

- Demo1PlainThreadLocal: InheritableThreadLocal across thread models (no Spring)
- Demo2AsyncVirtualThreads: @Async on a virtual-thread SimpleAsyncTaskExecutor
  (DelegatingSecurityContextExecutor vs ContextPropagatingTaskDecorator)
- Demo3StructuredConcurrency: StructuredTaskScope.fork() propagation
- Demo4ExecutorWrapping: DelegatingSecurityContextExecutorService/Executor/
  AsyncTaskExecutor on a classic pooled platform-thread executor -- the post's
  "Using @Async" / "Using ExecutorService" / "Using CompletableFuture" sections
- Demo5ReactiveContext: ReactiveSecurityContextHolder vs. ThreadLocal across a
  Reactor scheduler hop -- the post's WebFlux/getProfile() section
- Demo6ScheduledSystemIdentity: DelegatingSecurityContextTaskScheduler's actual
  per-call capture semantics (confirmed via bytecode before writing the demo) and
  the createSystemContext() pattern -- the post's scheduled-tasks section
- Demo7ServletFilterPersistence: SecurityContextHolderFilter (load-only) vs.
  SecurityContextPersistenceFilter (load+auto-save), against real filter instances
  and a real HttpSession -- the post's servlet-environment section
- SecurityContextPropagationContractTest: 10 JUnit tests pinning the above as
  assertions instead of printed lines, including a TestSecurityContextHolder-based
  test reproducing the post's own "Testing Security Context Propagation" section

Thirteen edge cases discovered along the way are indexed in docs/08 with links into
the chapter that reproduces each one -- a reused pool worker NOT leaking under the
Delegating* wrappers (unlike Demo1's InheritableThreadLocal), the common ForkJoinPool
trap, why there's no DelegatingSecurityContextStructuredTaskScope and never will be,
a real NullPointerException from Reactor's map() hit while writing the reactive test,
per-call (not per-construction) context capture in DelegatingSecurityContextTaskScheduler,
and the precise load-vs-save split between the two servlet filters, among others.

docs/01-08 are numbered, cross-linked chapters with prev/next navigation; README
indexes all demos, chapters, captured output, and the edge-case list. scripts/run-all.sh
regenerates every docs/output/*.txt and the test suite output from one command.

docs/output/demo4.txt

@@ -0,0 +1,9 @@
+=== Demo 4: Executor/ExecutorService/AsyncTaskExecutor wrapping on a pooled platform thread ===
+A) raw ThreadPoolExecutor, no wrapper: NO AUTHENTICATION (lost)
+B) DelegatingSecurityContextExecutorService.execute(...): authenticated as bob
+B2) DelegatingSecurityContextExecutorService.submit(Callable): authenticated as bob
+EDGE) task 1 on possibly-reused worker: authenticated as carol-task1
+EDGE) task 2, same pool, different caller context: authenticated as dave-task2  <-- correct, NOT stale, unlike plain InheritableThreadLocal on a reused worker
+C) DelegatingSecurityContextExecutor + CompletableFuture.supplyAsync: authenticated as erin
+C2) default CompletableFuture executor (common ForkJoinPool), no wrapper: NO AUTHENTICATION (lost)
+D) DelegatingSecurityContextAsyncTaskExecutor wrapping ThreadPoolTaskExecutor: authenticated as grace