ui: add dev.perfetto.ScatterPlot plugin and __intrinsic_scatter_mipmap operator

Adds a GPU-accelerated 2D scatter plot page plugin (dev.perfetto.ScatterPlot)
over any trace processor table or SQL query, backed by a new trace processor
2D level-of-detail virtual table operator (__intrinsic_scatter_mipmap).

Trace processor (src/trace_processor/plugins/scatter_mipmap_operator):
- Builds a 2D Morton-ordered spatial index (2-pass 11-bit radix sort + level-T
  prefix table, 24 B/point SoA) once on CREATE VIRTUAL TABLE and answers
  viewport queries (cols x rows cells) in O(cells) via aligned prefix-table
  lookup, with a quadtree DFS fallback for arbitrary unaligned viewports.
- Returns one representative point per non-empty cell (id, x, y, c, count),
  plus hidden aggregate columns (out_count, out_x_min/max, out_y_min/max,
  out_c_min/max) so dataset bounds are read without a separate SQL scan.
- Direct dataframe column ingestion for Int64, Double, Uint32, Int32 and
  StringPool::Id columns, with a SQL projection fallback for views and
  subqueries.
- Optional integer-N 6th argument computes top-N categorical frequencies in
  pass 1 (mapping top N to 0..N-1, remaining non-null values to N 'Other',
  and NULL to -1), with a cols=0, rows=0 listing mode returning
  (out_cat_idx, out_cat_value, out_cat_count) for the legend.

UI plugin (ui/src/plugins/dev.perfetto.ScatterPlot):
- WebGL2 renderer with hi/lo float32 splitting for nanosecond-scale timestamps
  and a 1D RGBA8 color LUT texture (viridis continuous + 20-category palette).
- Power-of-two aligned viewport quantization with 15% padding so pans within
  the padded region do not re-query trace processor.
- Cursor-anchored smooth wheel zoom, drag pan, Shift+drag box selection with
  lazy SQL DataGrid aggregation and query-page handoff, Popper-backed
  CursorTooltip, and a collapsible DrawerPanel embedding ThreadSliceDetailsPanel
  for pinned slices and Tree/Timestamp/DurationWidget for generic tables.