release: bump 0.9.3 -> 0.9.4 (#326)

Packaging fix for the `0.9.3` data layer.

`0.9.3` published to crates.io, Maven Central, NuGet, npm, and the Go mirror, but
the **Linux Python wheels failed to build**, so `Publish to PyPI` was skipped and
no GitHub Release was attached. Root cause: the `live-binance` data layer links
`native-tls` -> `openssl-sys`, and the `manylinux` / `musllinux` wheel-build
containers do not ship the system OpenSSL headers. The native macOS and Windows
wheels were unaffected (system TLS), which is why CI — running Python natively on
the runners, where OpenSSL is present — stayed green.

### Changes
- **`ci`**: install the OpenSSL headers inside the wheel container via
  maturin-action's `before-script-linux` (`openssl-devel` on `manylinux`,
  `openssl-dev` on `musllinux`) before maturin compiles. No library code changed.
- **`release: bump 0.9.3 -> 0.9.4`**: version-string bump across the manual
  touchpoints + `CHANGELOG` `[0.9.4]`.

`0.9.4` is functionally identical to `0.9.3`; it exists only because the already-
published registries cannot re-release `0.9.3`. PyPI publishes for the first time
starting at `0.9.4` (it skips `0.9.3`).

examples/java/pom.xml

@@ -6,7 +6,7 @@
 
   <groupId>org.wickra.examples</groupId>
   <artifactId>wickra-examples</artifactId>
-  <version>0.9.3</version>
+  <version>0.9.4</version>
   <packaging>jar</packaging>
 
   <name>Wickra Java examples</name>
@@ -21,7 +21,7 @@
     <dependency>
       <groupId>org.wickra</groupId>
       <artifactId>wickra</artifactId>
-      <version>0.9.3</version>
+      <version>0.9.4</version>
     </dependency>
   </dependencies>