Add the filter-chain module

Companion project for "The Spring Security Filter Chain Explained". A real
Spring Boot 4.1.1 servlet application whose scenarios are Spring profiles, plus
a diagnostic controller that prints the live FilterChainProxy, the reflected
FilterOrderRegistration table, and the servlet container's own registrations.

Twelve captured transcripts under docs/output/, nine cross-linked doc chapters,
21 assertions.

Also fixes a broken relative link in method-security/docs/01: the cross-module
reference to context-propagation/README.md needed two levels up, not one.

filter-chain/pom.xml

@@ -0,0 +1,67 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0">
+  <modelVersion>4.0.0</modelVersion>
+
+  <!-- Inheriting spring-boot-starter-parent (rather than importing the BOM) so that this
+       module gets Boot's own compiler settings, including <parameters>true</parameters>.
+       Every version below is managed by the parent; nothing here is pinned by hand. See
+       docs/01-the-two-proxies.md for why this module is a real servlet app while the other
+       modules in this repository are plain main() classes. -->
+  <parent>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-parent</artifactId>
+    <version>4.1.1</version>
+    <relativePath/>
+  </parent>
+
+  <groupId>com.ankurm</groupId>
+  <artifactId>filter-chain-verify</artifactId>
+  <version>1.0</version>
+  <packaging>jar</packaging>
+
+  <properties>
+    <java.version>25</java.version>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-web</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-security</artifactId>
+    </dependency>
+    <dependency>
+      <!-- Present ONLY so docs/06-observability-changes-the-trace.md can show what an
+           ObservationRegistry bean does to FilterChainProxy's decorator. Remove it and the
+           TRACE output in docs/output/demo3-trace-annotated.txt changes shape. -->
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-webmvc-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-security-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>