perf(console): tail-cap large RPC results without a full copy

A large RPC result was fully split into lines and copied before being
handed to the console, even though only the last few lines could survive
the model's line cap.

- The formatter now takes a line cap and keeps only the last N lines,
  located by a backward newline scan, so a huge result is never fully
  split or copied. JSON classification still keys off the first
  character of the whole result.
- It works over a string view and moves line text through, and fold
  spans are computed over the formatted lines directly instead of a
  separate vector of copies.
- getline semantics are preserved: a trailing newline adds no empty
  line and an empty result yields no lines.

Tests: getline parity for empty, trailing-newline and blank-line inputs,
the backward tail cap, and JSON classification and fold spans over a
capped result.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/console_tab.cpp

@@ -2052,15 +2052,15 @@ void ConsoleTab::renderCommandsPopup(ConsoleCommandExecutor& exec)
 
 void ConsoleTab::addFormattedResult(const std::string& result, bool is_error)
 {
-    const std::vector<ConsoleResultLine> resultLines = FormatConsoleRpcResultLines(result, is_error);
+    // Only the last maxLines lines can survive the model's cap, so a huge result is tail-capped
+    // up front instead of being fully split and copied.
+    std::vector<ConsoleResultLine> resultLines =
+        FormatConsoleRpcResultLines(result, is_error, model_.maxLines());
 
     // Precompute JSON fold spans over the whole block so each opener line carries the offset
     // to its matching closer (the spans are relative, so they stay valid as the model's line
     // cap evicts from the front). The block is ingested under one lock (ingestBatch).
-    std::vector<std::string> texts;
-    texts.reserve(resultLines.size());
-    for (const auto& rl : resultLines) texts.push_back(rl.text);
-    const std::vector<int> foldSpans = ComputeConsoleFoldSpans(texts);
+    const std::vector<int> foldSpans = ComputeConsoleResultFoldSpans(resultLines);
 
     std::vector<ConsoleModelLine> block;
     block.reserve(resultLines.size());
@@ -2075,7 +2075,7 @@ void ConsoleTab::addFormattedResult(const std::string& result, bool is_error)
             case ConsoleResultLineRole::Result: break;
         }
         ConsoleModelLine ml;
-        ml.text = resultLines[i].text;
+        ml.text = std::move(resultLines[i].text);
         ml.channel = channel;
         ml.foldSpan = foldSpans[i];
         block.push_back(std::move(ml));