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/CONFIGURATION.md

@@ -57,10 +57,13 @@ ordinary typing, code-lens probes, bulk scoped reference analysis, and Java Go t
 retained, with a shared 100,000-association retention cap. Records contain property names, element handles and source locations;
 ASTs and bindings are discarded after indexing. A lexical prefilter avoids AST
 construction for ordinary Java units, but every accepted association is resolved
-semantically. Warm requests reuse the snapshot. Java model/reconcile changes
-invalidate it conservatively, including dependent-type changes; open working-copy
-content fingerprints also prevent stale answers before reconciliation. This is
-**snapshot invalidation**, not a fully incremental Java semantic index.
+semantically. Warm requests reuse the snapshot. Java model changes (sources on
+disk, classpath, archives) invalidate it conservatively, including dependent-type
+changes; a fingerprint of working copies with unsaved changes covers edits before
+they are saved. Opening or closing an editor, reconciling, building, and non-Java
+resource changes (configuration files are parsed by the client; metadata files are
+checked by timestamp) keep it. This is **snapshot invalidation**, not a fully
+incremental Java semantic index.
 
 Metadata/JAR timestamps are checked before reuse. The client caches parsed disk
 configuration files by modification time and size (at most 100,000 cached keys), overlays open buffers, and