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_lower.bux

@@ -3423,6 +3423,7 @@ module HirLower {
         f.name = decl.strValue;
         f.isPublic = decl.isPublic;
         f.checkedFunc = ctx.checkedFunc;
+        f.sourceFile = decl.sourceFile;
         f.paramCount = decl.paramCount;
         f.param0 = bux_alloc(sizeof(HirParam)) as *HirParam;
         Lcx_LowerParam(f.param0, &decl.param0, ctx);