feat(selfhost): multi-file #line paths from Decl.sourceFile

Stamp each merged .bux path onto decls, propagate to HirFunc, and emit
#line N \"path\" per function without requiring BUX_DEBUG_FILE.

- Decl.sourceFile + Cli_StampSourceFile on merge/project parse
- HirFunc.sourceFile; CBE switches currentFile per function
- Stdlib and multi-file user packages get distinct paths automatically

src/hir.bux

@@ -115,6 +115,8 @@ module Hir {
         envStructName: String;
         envInstanceName: String;
         checkedFunc: bool;
+        // Absolute/relative path for #line maps (multi-file selfhost)
+        sourceFile: String;
     }
 
     // ---------------------------------------------------------------------------