PDF gen: capture lualatex stdout/stderr into the error message

The detailed LaTeX error (missing .sty, missing \includegraphics
target, …) was being lost: we only redirected stdin, and the engine
writes its diagnostics to the .log file we delete in cleanup. So a
generation failure surfaced only as the terse "Emergency stop" in the
journal, with no clue what actually failed.

Redirect stdout and stderr, and on non-zero exit include them in
GenerationErrorMessage so the caller sees the real engine diagnostic.
Keep the .log file on failure (only clean it on success) so it can be
inspected alongside.

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

src/Yavsc.Api/Helpers/TeXHelpers.cs

@@ -134,7 +134,13 @@ namespace Yavsc.Helpers
             // directory. lualatex is a file-based engine — it cannot emit
             // the PDF on stdout — so the pdf lands at its final destination
             // and we only clean the transient aux/log artifacts beside it.
+            //
+            // We capture stdout/stderr so a LaTeX error (missing .sty,
+            // missing \includegraphics target, …) is surfaced in
+            // GenerationErrorMessage instead of being buried in the .log
+            // we then delete.
             int exitCode;
+            string stdout = null, stderr = null;
             try
             {
                 using (Process p = new Process())
@@ -147,14 +153,21 @@ namespace Yavsc.Helpers
                         Arguments = $"-interaction=nonstopmode -halt-on-error"
                             + $" -jobname=\"{name}\" -output-directory=\"{billdir}\"",
                         RedirectStandardInput = true,
+                        RedirectStandardOutput = true,
+                        RedirectStandardError = true,
                     };
                     p.Start();
+                    // Write the TeX source, then drain stdout/stderr. Read
+                    // them after WaitForExit — lualatex's output is small
+                    // (a few KB), well under the pipe buffer, so no deadlock.
                     using (var stdin = p.StandardInput)
                     {
                         stdin.Write(Model.TeXSource);
                     }
                     p.WaitForExit();
                     exitCode = p.ExitCode;
+                    stdout = p.StandardOutput.ReadToEnd();
+                    stderr = p.StandardError.ReadToEnd();
                 }
             }
             catch (Exception ex)
@@ -164,15 +177,24 @@ namespace Yavsc.Helpers
             }
 
             if (exitCode != 0 && errorMsg == null)
-                errorMsg = $"Pdf generation failed with exit code: {exitCode}";
+            {
+                // Surface the engine's own diagnostics (stderr carries the
+                // "! Emergency stop" / "! LaTeX Error: …" lines; stdout has
+                // the transcript banner) so the caller can see why it failed
+                // without having to find the (soon-deleted) .log file.
+                var diag = string.IsNullOrWhiteSpace(stderr) ? stdout : stderr;
+                errorMsg = $"Pdf generation failed with exit code {exitCode}: {diag}";
+            }
             else if (!fo.Exists && errorMsg == null)
                 errorMsg = "Pdf generation produced no output";
 
             // Remove the LaTeX build artifacts left beside the pdf so only
-            // <name>.pdf persists in the bills directory.
+            // <name>.pdf persists in the bills directory. Keep the .log on
+            // failure so it can be inspected.
             foreach (var ext in new[] { ".aux", ".log", ".out", ".fls",
                         ".fdb_latexmk", ".synctex.gz", ".toc" })
             {
+                if (ext == ".log" && !fo.Exists) continue;
                 var f = new FileInfo(System.IO.Path.Combine(billdir, name + ext));
                 if (f.Exists) { try { f.Delete(); } catch { } }
             }