build(isthmus)!: stop publishing the isthmus fat jar

[nielspardon]

The :isthmus module applies the shadow plugin only to build an uber jar (isthmus-<version>-all.jar), whose manifest still declares the long-removed io.substrait.isthmus.PlanEntryPoint as its Main-Class. The CLI was extracted into the :isthmus-cli module (built as a GraalVM native image) years ago, so:

• nothing in the repo consumes this jar, and
• :isthmus shades/relocates nothing of its own — the ANTLR-runtime relocation that prevents version clashes with consumers lives in :core (relocate("org.antlr.v4.runtime", …)) and is untouched here.

The GradleUp shadow plugin injects a shadowRuntimeElements variant into the java component by default (addShadowVariantIntoJavaComponent), so from(components["java"]) has been publishing this fat jar to Maven Central as an all-classified artifact on every release. Confirmed via the publication's Gradle module metadata:

before                                   after
apiElements       -> isthmus.jar         apiElements       -> isthmus.jar
runtimeElements   -> isthmus.jar         runtimeElements   -> isthmus.jar
shadowRuntimeElements -> isthmus-all.jar  (removed)
javadocElements   -> isthmus-javadoc.jar javadocElements   -> isthmus-javadoc.jar
sourcesElements   -> isthmus-sources.jar sourcesElements   -> isthmus-sources.jar

Change

Remove the shadow plugin (and its shadowJar/build wiring) from :isthmus. The classes { dependsOn(":core:shadowJar") } line is retained — compiling isthmus needs core's relocated jar.

Impact

Standard consumers (implementation("io.substrait:isthmus:<version>")) resolve runtimeElements (the plain jar) and are unaffected. Only a consumer that explicitly requested the all classifier / shadowed variant is affected — see the breaking-change note below.

BREAKING CHANGE: the isthmus fat jar (isthmus-<version>-all.jar — the all classifier / shadowRuntimeElements variant) is no longer published to Maven Central. Consumers that referenced it explicitly should depend on the regular isthmus artifact and its transitive dependencies instead.

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[bestbeforetoday]

Change looks OK to me. Really nobody should have been referencing the isthmus--all.jar anyway, so it doesn't obviously seem like a concern.