Release SCI 0.15.59

[whilo]

Summary

• release the forkable runtime and continuation-context work as SCI 0.15.59
• move the current Unreleased changes under a versioned changelog heading
• update the embedded SCI version resource

Verification

• upstream forkable-worlds CI is green across JVM, ClojureScript/Node, ClojureDart, native, self-hosted, and libsci
• lein pom generates org.babashka:sci:0.15.59
• lein jar contains matching SCI_VERSION and pom.properties
• git diff --check

An independent release review found no medium-or-higher issues.

Release ordering

This must be published before the stacked Spindel SCI-world integration switches from its temporary commit pin to org.babashka/sci:0.15.59. Publishing automation is master-only, so the final release commit must ultimately land on the publishing repository's master branch.