refactor(ext-auth): extract flash-ext-auth-core out of flash-ext-oidc

flash-ext-oidc has always held two things: the OpenID Connect protocol, and a
session/claims/authorization layer that is generic and was only ever fed by one
source. This splits them along Flash's own <domain>-core convention, the same
shape cache-core, data-core and view-core already use.

flash-ext-auth-core gets what never referenced the protocol — @Authenticated,
@RolesAllowed, @ScopesAllowed, ClaimsHolder, the claim matching, and the policy
compiled from annotations — under generic names: OidcUser is Claims, since it
never was more than a typed view over a claims map, and OidcAuthPolicy is
AuthPolicy. It was package-private while being the parameter type of a public
method, so the move also fixes that.

The new seam is CredentialSource: it resolves a request's claims, or rejects the
request the way its protocol says to. AuthMiddleware publishes the result and
matches roles and scopes against it. ClaimsHolder's writers stay package-private
— an implementation produces claims and core publishes them, so nothing outside
this module can put claims on a request that did not carry them.

flash-ext-oidc keeps discovery, JWKS, PKCE, the token endpoint, the login and
callback routes and the OpenAPI oauth2 contributor, and now registers
OidcCredentialSource. flash-ext-mcp still keys McpSecurity on finding that type
and not on AuthMiddleware: REQUIRED has to keep meaning "a real authorization
server is protecting this endpoint", not "something authenticates here".

The middleware key moves with the mechanism: flash.oidc.policy -> flash.auth.policy.

Breaking for consumers: imports move to dev.relism.flash.ext.auth, OidcMiddleware
becomes AuthMiddleware, ClaimsHolder.user()/get() become current()/map().

flash-extensions/flash-ext-auth-core/pom.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>dev.relism</groupId>
+        <artifactId>flash-extensions</artifactId>
+        <version>2.1.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>flash-ext-auth-core</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>dev.relism</groupId>
+            <artifactId>flash</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+        </dependency>
+    </dependencies>
+</project>