release: bump 0.8.6 -> 0.8.7 (#263)

Version bump `0.8.6` → `0.8.7`.

### Added
- R binding: a *Getting started* vignette and a synthetic `sample_ohlcv` example dataset, giving new users a runnable, self-contained walkthrough and populating the R-universe Articles and Datasets tabs. The vignette's code is exercised in CI so a broken example is caught before the published build. (#262)

Bump touches the manual release touchpoints only; docs/webpage version strings are left to `sync-about.yml` on the tag.

examples/java/pom.xml

@@ -6,7 +6,7 @@
 
   <groupId>org.wickra.examples</groupId>
   <artifactId>wickra-examples</artifactId>
-  <version>0.8.6</version>
+  <version>0.8.7</version>
   <packaging>jar</packaging>
 
   <name>Wickra Java examples</name>
@@ -21,7 +21,7 @@
     <dependency>
       <groupId>org.wickra</groupId>
       <artifactId>wickra</artifactId>
-      <version>0.8.6</version>
+      <version>0.8.7</version>
     </dependency>
     <!-- Only the network examples (fetch_btcusdt) parse JSON. -->
     <dependency>