Bump BAST FORMAT_REVISION to 6

RequireStatement wire format changed (added condition-kind byte)
and ReplyStatement is a new subtype (15). Old BAST files are
incompatible with this revision.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: reid-spencer

language/shared/src/main/scala/com/ossuminc/riddl/language/bast/package.scala

@@ -58,7 +58,7 @@ package object bast {
     * with revision 0 (pre-check era) will be rejected with a
     * clear message.
     */
-  val FORMAT_REVISION: Short = 5
+  val FORMAT_REVISION: Short = 6
 
   /** Magic bytes for BAST file identification: "BAST" */
   val MAGIC_BYTES: Array[Byte] = Array('B'.toByte, 'A'.toByte, 'S'.toByte, 'T'.toByte)