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/ast.bux

@@ -299,6 +299,8 @@ module Ast {
         isDrop: int,             // @[Drop] attribute (0/1)
         isRelease: int,          // @[Release] attribute (0/1)
         isConst: int,            // const func (0/1)
+        // Source file path for #line / diagnostics (multi-file projects)
+        sourceFile: String,
         // Names
         strValue: String,        // decl name
         strValue2: String,       // interface name, dll name, module path
@@ -418,6 +420,7 @@ module Ast {
 
     func Ast_MakeDecl(kind: int, line: uint32, col: uint32) -> Decl {
         return Decl { kind: kind, line: line, column: col, isPublic: false,
+            sourceFile: "",
             isAsync: false, isChecked: 0, isDrop: 0, isRelease: 0, isConst: 0,
             strValue: "", strValue2: "",
             typeParam0: "", typeParam1: "", typeParamCount: 0,

src/c_backend.bux

@@ -1407,7 +1407,7 @@ module CBackend {
         cbe.lastDebugLine = 0;
         cbe.emitDebugLines = true;
         cbe.currentFile = "";
-        // Optional: BUX_DEBUG_FILE sets the #line path for gdb (selfhost has no multi-file loc yet)
+        // Optional override for all #line paths (default: per-func Decl.sourceFile)
         let dbgFile: String = bux_getenv("BUX_DEBUG_FILE");
         if dbgFile != null as String && !String_Eq(dbgFile, "") {
             cbe.currentFile = dbgFile;
@@ -1696,6 +1696,17 @@ module CBackend {
                 i = i + 1;
                 continue;
             }
+            // Per-function source path for multi-file #line (overrides BUX_DEBUG_FILE only if set)
+            if !String_Eq(mod.funcs[i].sourceFile, "") {
+                cbe.currentFile = mod.funcs[i].sourceFile;
+            }
+            cbe.lastDebugLine = 0;
+            // #line before the function itself
+            if cbe.emitDebugLines && !String_Eq(cbe.currentFile, "") {
+                StringBuilder_Append(&cbe.sb, "#line 1 \"");
+                StringBuilder_Append(&cbe.sb, cbe.currentFile);
+                StringBuilder_Append(&cbe.sb, "\"\n");
+            }
             CBE_EmitFuncDecl(cbe, &mod.funcs[i]);
             StringBuilder_Append(&cbe.sb, " {\n");
             // Capturing closure thunk: materialize env from fat-func env pointer

src/cli.bux

@@ -611,6 +611,27 @@ func Cli_CollectStdlibImports(mod: *Module, outPaths: *String, maxCount: int, st
     return count;
 }
 
+/// Stamp sourceFile on a decl and nested module items (for multi-file #line).
+func Cli_StampSourceFile(decl: *Decl, path: String) {
+    if decl == null as *Decl { return; }
+    decl.sourceFile = path;
+    if decl.kind == dkModule {
+        var inner: *Decl = decl.childDecl1;
+        while inner != null as *Decl {
+            Cli_StampSourceFile(inner, path);
+            inner = inner.childDecl2;
+        }
+    }
+    // Impl methods
+    if decl.kind == dkImpl {
+        var m: *Decl = decl.childDecl1;
+        while m != null as *Decl {
+            Cli_StampSourceFile(m, path);
+            m = m.childDecl2;
+        }
+    }
+}
+
 func Cli_MergeFileInto(target: *Module, path: String, skipNames: *String, skipCount: int) -> int {
     if !FileExists(path) {
         Print("Error: stdlib file not found: ");
@@ -623,6 +644,12 @@ func Cli_MergeFileInto(target: *Module, path: String, skipNames: *String, skipCo
     if Lexer_DiagCount(lex) > 0 { return 0; }
     let mod: *Module = Parser_Parse(lex.tokens, lex.tokenCount);
     if mod == null as *Module { return 0; }
+    // Tag every decl from this file for #line maps
+    var stamp: *Decl = mod.firstItem;
+    while stamp != null as *Decl {
+        Cli_StampSourceFile(stamp, path);
+        stamp = stamp.childDecl2;
+    }
 
     var added: int = 0;
     var decl: *Decl = mod.firstItem;
@@ -1431,6 +1458,12 @@ func Cli_BuildProject(projectDir: String, targetTriple: String, isRelease: bool)
             PrintLine(path);
             return 1;
         }
+        // Tag decls with this source path for multi-file #line
+        var stampUser: *Decl = mod.firstItem;
+        while stampUser != null as *Decl {
+            Cli_StampSourceFile(stampUser, path);
+            stampUser = stampUser.childDecl2;
+        }
         // Merge declarations from this module into userMerged
         var decl: *Decl = mod.firstItem;
         var fileDeclCount: int = 0;

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;
     }
 
     // ---------------------------------------------------------------------------

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);