Extend API compatibility check to detect removed KMP targets as breaking

[hdcodedev]

Problem

The API compatibility check (apiCompatibilityCheck in buildSrc/src/main/kotlin/charts.api-compatibility.gradle.kts) uses japicmp to diff JVM jars only against a baseline ref (.github/api-compatibility-baseline.txt). It cannot detect the removal of a Kotlin Multiplatform target (e.g. dropping the legacy js target in #511), even though a target removal is a breaking change for consumers on that target.

Current behavior

• Only JVM binary/source compatibility is verified.
• Removed targets (or API drift on wasmJs/iOS metadata/klibs) are invisible to CI.
• Breaking changes are surfaced only via release-note migration fragments, which are not enforced.

Proposed solutions

Option A (cheap, targeted): Compare the published target set per module (e.g. kotlin.targets or publication names) between the baseline ref and the current tree, and feed the existing breaking-change label gating in .github/workflows/api-compatibility.yml — fail when a target disappears without the label.

Option B (thorough, larger effort): Adopt the Kotlin binary-compatibility-validator plugin, which produces per-target .api dumps, detects removed targets (dump file disappears) and API drift on non-JVM targets. Overlaps with the existing japicmp setup and requires an api/*.api migration.

Recommendation

Option A is a proportionate safeguard for the current small, intentional target set; Option B is the long-term end-state if per-target API drift protection is wanted.

Acceptance criteria

• A removed KMP target without the breaking-change label fails the API compatibility job.
• Intentional removals pass when the label (and migration fragment) are present.
• Existing label/categorization output in the workflow is preserved or improved.