perf(console): extend the filtered view incrementally

With any console filter on, each batch of new output rescanned all N
lines to rebuild the visible set. A text filter lowercase-copied the
needle and every line to test it: O(total characters) and 2N
allocations per batch.

When the filter is unchanged and the model only appended or evicted
lines since the last pass, the visible indices drop the evicted lines
and shift down, and only the new lines are tested. The result equals
a full rescan, since line text and channel never change and the
filtered view has no folds. A filter change, Clear, or a model window
that doesn't follow on from the last one still rescans.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/windows/console_output_model.h

@@ -2,7 +2,10 @@
 
 #include "console_channel.h"
 
+#include <cstdint>
+#include <functional>
 #include <string>
+#include <vector>
 
 namespace dragonx {
 namespace ui {
@@ -20,5 +23,13 @@ bool consoleLinePassesFilter(const std::string& lineText,
                              ConsoleChannel channel,
                              const ConsoleOutputFilter& filter);
 
+// Incrementally maintain a FLAT (filtered, fold-free) visible set across model appends/evictions. `visible` holds
+// ascending raw indices computed for the absolute window [oldFirst, oldEnd); the model now spans
+// [newFirst, newFirst + n). Drops evicted indices, rebases the survivors and appends every new raw index in
+// [oldEnd - newFirst, n) for which passes(i) is true. Returns false (visible untouched) when the windows do not
+// chain; the caller then rescans. Only valid while the filter is unchanged (line text/channel are immutable).
+bool ExtendFlatVisibleIndices(std::vector<int>& visible, std::uint64_t oldFirst, std::uint64_t oldEnd,
+                              std::uint64_t newFirst, int n, const std::function<bool(int)>& passes);
+
 } // namespace ui
 } // namespace dragonx