Add skainet-backend-api to BOM (#400)

The neutral backend api module landed in #470 as the integration
seam for future backends (IREE, Metal, NPU, the NNAPI-Amlogic
sibling repo) but it was never added to the BOM's version-
alignment constraints. Java / JVM consumers that depend on the
BOM were therefore not getting a pinned version for
skainet-backend-api, so anyone referencing the module from a
Maven / Gradle project had to either spell out the version
manually or drop the BOM reliance for that coordinate.

Adding the missing `api(project(":skainet-backends:skainet-backend-api"))`
constraint groups it with skainet-backend-cpu under the
backend section. BOM still builds clean.

First of five commits polishing the Java / JVM consumption
story for the upcoming 0.19.0 release. See #400.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michalharakal

skainet-bom/build.gradle.kts

@@ -15,7 +15,8 @@ dependencies {
         // Core language module
         api(project(":skainet-lang:skainet-lang-core"))
 
-        // CPU backend
+        // Backend abstraction + CPU backend
+        api(project(":skainet-backends:skainet-backend-api"))
         api(project(":skainet-backends:skainet-backend-cpu"))
 
         // IO modules