Sync the MiMa binary-compatibility check

[halotukozak]

Moves the prototype validated on halotukozak-com/commons#38 into the shared config.

Recap of the prototype

• .mima/bin-compat-check.scala — no MiMa plugin exists for scala-cli, so this calls `com.typesafe::mima-core`'s API directly (Scala 2.13 script, same pattern as `.scoverage/report.sc`). Compares the last released JAR against a freshly built `--library` JAR.
• `shared/mima.yml` — resolves the baseline from the latest `v*` tag (via `publish.organization`/`publish.name` in `project.scala`, so it's fully generic), report-only: manages a `needs-major` label on a PR when there's a real backward incompatibility, never fails the build. Removes the label again once a later push is compatible.
• Only backward compatibility gates the label — forward is printed for context but always shows "problems" whenever you add API (expected for any minor bump), so it can't be a failure signal.

Verified end-to-end on commons#38: label appears on a real backward break, disappears once reverted.

Sync targets

commons/made/mcodec/mrpc — same exclusions as `ci.yml`/`docs.yaml`:

• regex — would need its own `--exclude "bench/**"` wired into the `scala-cli --power package`/`compile` calls first (same JMH-benchmark issue as before).
• alpaca — Mill, entirely different build/publish/classpath story. Being done separately, not through this sync.

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