refactor(vortex-web): drill-down treemap with a single path header

Replace the nested treemap (and its per-container header strips, the source of
the overlapping labels) with a drill-down box: the box shows the current
layout's child blocks one level deep, each a single labelled tile sized by its
subtree bytes. A header bar at the top is the current path — click a block to
drill in, click a path segment to drill back out. Because every visible tile is
one level deep, labels never collide. Hover still drives the tooltip, sidebar,
and data sample; selection stays in sync with the tree panel. Drops the
pan/zoom transform and sticky-header machinery.

Signed-off-by: Robert <robert@spiraldb.com>

Author: robert3005

vortex-web/README.md

@@ -4,24 +4,20 @@ A web UI for exploring Vortex data files, built with React, TypeScript, Tailwind
 
 ## Treemap Explorer
 
-The **Treemap** view (toggle in the header, next to the theme picker) is a map
-of a file's physical blocks. Layouts subdivide the canvas first, flat layouts
-subdivide further into their array-encoding buffers, and every tile is sized by
-its byte footprint and coloured by dtype, matching the swimlane and detail
-treemap. The map is zoomable (scroll), pannable (drag), and supports
-double-click zoom-to-fit; selection and hover stay in sync with the tree panel.
-It follows the app theme — dark by default, matching explore.vortex.dev — and
-the light theme via the theme picker.
+The **Treemap** view (toggle in the header, next to the theme picker) is a
+drill-down map of a file's physical blocks. The box shows the current layout's
+blocks, sized by byte footprint and coloured by dtype to match the swimlane and
+detail treemap; a header bar at the top is the current path. Click a container
+block to drill into it, and click a segment of the path header to drill back
+out. It follows the app theme — dark by default, matching explore.vortex.dev —
+and the light theme via the theme picker.
 
 ![Treemap explorer overview](docs/img/treemap-explorer-overview.png)
 
-Each container layout gets a title strip in its padding band, and those strips
-are sticky: when a container scrolls above the top of the view its strip pins
-just below its parent's, so the ancestor strips stack into the current path as
-you pan and zoom. Hovering or selecting a tile shows its physical statistics —
-data and metadata bytes, percentage of the file, encoded bytes-per-row density,
-rows, and segment/buffer counts — in a floating tooltip and the sidebar,
-alongside a data sample for the selected block.
+Hovering a block shows its physical statistics — data and metadata bytes,
+percentage of the file, encoded bytes-per-row density, rows, and segment/buffer
+counts — in a floating tooltip; the sidebar and data sample track the selected
+block, and selection stays in sync with the tree panel.
 
 ![Hovering a tile shows physical statistics](docs/img/treemap-explorer-hover.png)