Parallelize terser minification in processTerserFullLink

[Copilot]

Terser invocations in processTerserFullLink were sequential — each JS file was minified one at a time, leaving CPU and I/O capacity on the table for any project with more than one output chunk.

Changes

• processTerserFullLink: Replace jsFiles.foreach with jsFiles.map { f => Future { ... } } + Await.result(Future.sequence(...), Duration.Inf) so all JS files are minified concurrently
• Imports: Add scala.concurrent.{Await, Future} and scala.concurrent.duration.Duration; reuse the existing package-level implicit val ec from refresh_plugin.scala (no new dependency)

// Before
jsFiles.foreach { f =>
  os.proc("terser", ...).call(...)
}

// After
val futures = jsFiles.map { f =>
  Future { os.proc("terser", ...).call(...) }
}
Await.result(Future.sequence(futures), Duration.Inf)

Failure semantics are unchanged — if any terser process exits non-zero, the build fails.