perf(java): keep the configuration snapshot across editor activity

The configuration index was invalidated by every Java element event,
including POST_RECONCILE, and its working-copy fingerprint covered every
open editor. Opening the navigation target, closing a tab, or saving
application.yml was enough to make the next Find Usages or YAML key
navigation rebuild the whole project index.

Invalidation now follows what the index depends on:
- POST_CHANGE deltas only, ignoring working-copy open/close, AST-only
  flags and non-Java resource content on projects, roots and packages
  (configuration files are parsed by the client, metadata files are
  stamp-checked).
- The fingerprint covers working copies with unsaved changes only.

Each snapshot reports a generation so the real-server harness can assert
that opening/closing editors, a full build and a saved edit keep it warm,
while a source added on disk still invalidates it.

Bundle 1.2.1 (installed as java-usages-1.2.1.jar so OSGi does not reuse
the cached 1.2.0), reproducible SHA-256
cbae3e3a0bcf2818b4cc9d98998bd9b81f92e1ae7803dc757134485f9776a4e1.

plugins/java-usages/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: OpenIDEA Java usages
 Bundle-SymbolicName: ai.openidea.usages;singleton:=true
-Bundle-Version: 1.2.0
+Bundle-Version: 1.2.1
 Bundle-RequiredExecutionEnvironment: JavaSE-21
 Require-Bundle: org.eclipse.jdt.ls.core,org.eclipse.jdt.core,
  org.eclipse.jdt.core.manipulation,org.eclipse.core.runtime,