Fix prettify include paths to be relative to containing file

Include text was using the CWD-relative path from the AST, which
produced wrong paths at every nesting level. Now computes the
include path relative to the current output file by stripping
the current file's directory prefix from the included file's
output path. Works correctly for both top-level and nested files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: reid-spencer

passes/shared/src/main/scala/com/ossuminc/riddl/passes/prettify/PrettifyVisitor.scala

@@ -417,8 +417,17 @@ class PrettifyVisitor(options: PrettifyPass.Options)(using PlatformContext) exte
     state.withCurrent { (rfe: RiddlFileEmitter) =>
       if !state.flatten then
         val url = include.origin
-        rfe.addLine(s"""include "${url.path}"""")
         val outputURL = state.toDestination(url)
+        // Compute include path relative to the current output file
+        val currentDir = rfe.url.path.lastIndexOf('/') match
+          case -1 => ""
+          case i  => rfe.url.path.substring(0, i)
+        val relativePath =
+          if currentDir.isEmpty then outputURL.path
+          else if outputURL.path.startsWith(currentDir + "/") then
+            outputURL.path.drop(currentDir.length + 1)
+          else outputURL.path
+        rfe.addLine(s"""include "$relativePath"""")
         val newRFE = RiddlFileEmitter(outputURL)
         state.pushFile(newRFE)
       end if