fix(render): bind identity into the epoch, titles into reserved strips, one layout per frame

Four review findings on the Task 7 renderer:

- InstanceStore::epoch() is derived from gl::Buffer::generation() -- advanced by
  every allocation, deletion and move -- plus the slot the instances were written
  to, so the orphan fallback of grow_persistent() can no longer leave the vertex
  array's attribute pointers bound to a deleted buffer: the epoch reports the new
  identity and upload() re-points them before the draw.
- PlotLayout reserves y_title_area/x_title_area outside axes (sized from the text
  metrics) and draw_titles() draws in them, so a title can no longer land on the
  data, whatever the envelope does.
- draw() computes plot.layout() once and hands it to the helpers, instead of six
  times per frame, each of which rebuilt the ticks and the grid.
- A refused fence is no longer dropped: mark_drawn() falls back to the orphan path
  and reports the failure rather than rotating through an unsynchronised slot.

New smoke cases force both refusals through the GL entry points (glMapBufferRange,
glFenceSync) and assert the pixels plus the live transfer mode; the title case
asserts the title colour is absent from the bands inside axes and present in both
strips. All three fail against the pre-fix sources.

include/plot/render/vertex_array.hpp

@@ -31,9 +31,14 @@ class LineVertexArray {
 
   void bind() const noexcept { vao_.bind(); }
   /// Records that the draw just issued read the instance buffer's current slot,
-  /// which is what lets the mapped path reuse it safely. A no-op on the baseline
-  /// path, so a caller always calls it the same way.
-  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+  /// which is what lets the mapped path reuse it safely. True in the baseline
+  /// mode, so a caller always calls it the same way. False only when the driver
+  /// refused the fence: the buffer has then fallen back to the baseline path, and
+  /// `persistent()` says so from the next call on.
+  bool mark_drawn() noexcept { return buffer_.mark_drawn(); }
+  /// True while this array's instance buffer is the mapped ring (see
+  /// `UploadMode`); a failure that falls back clears it.
+  bool persistent() const noexcept { return buffer_.persistent(); }
 
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }
@@ -57,7 +62,8 @@ class TextVertexArray {
   void upload(const f32* interleaved, u32 glyphs) noexcept;
 
   void bind() const noexcept { vao_.bind(); }
-  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+  bool mark_drawn() noexcept { return buffer_.mark_drawn(); }
+  bool persistent() const noexcept { return buffer_.persistent(); }
 
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }