BspBootstrapBridge

Daemon-side hook the BSP worker calls once per BSP request. The worker supplies the active command label, a meta-build compile reporter keyed by depth, and the body that consumes the freshly-bootstrapped evaluators; the daemon runs MillBuildBootstrap and threads the resulting evaluators (with the watches captured during the run, plus any error message) into body.

Declared as a SAM trait rather than a Scala 3 polymorphic-function alias so the worker can locate the method via Class.getMethod without depending on the polymorphic-function's erased shape (the previous [T] => (a, b, c) => T alias erased to scala.Function3 and caused a NoSuchMethodException at the reflection boundary when the lookup mistakenly used Function2).