fix(ui): stop dropping cached tiles when zooming away and back (#2196) (#2323)

- Retain tiles of on-screen pages at both eviction sites in
  crates/ui-egui/src/canvas.rs, so a pan or zoom that moves the viewport no
  longer discards tiles that were rendered sharp moments earlier.
- Stop dropping a zoom's tiles when zooming out far enough shows the page
  whole; that made returning to a visited zoom a cold miss, which re-blurred
  the page and re-interpreted it once per tile.
- Bound older-zoom retention by bytes (TILE_BYTES) as well as by count
  (OLD_TILES): a tile of a large scan can be 4096 squared, about 67 MiB, so a
  count alone ran past the tile allowance.
- Add returning_to_a_visited_zoom_reuses_its_cached_tiles, which fails on the
  reverted fix, and cite both view tests in view.deep-zoom-tiles; the entry
  stays partial because the 8192 px-per-side cap is untouched.

parity/acrobat-features.toml

@@ -842,8 +842,8 @@ milestone = "M3"
 status = "partial"
 commands = []
 tools = []
-evidence = ["crates/render/src/raster.rs::tiles_match_full_render", "crates/render/src/raster.rs::effective_scale_never_exceeds_the_caps"]
-notes = "capped at 8192 px per side (M3.3)"
+evidence = ["crates/render/src/raster.rs::tiles_match_full_render", "crates/render/src/raster.rs::effective_scale_never_exceeds_the_caps", "crates/ui-egui/tests/view.rs::returning_to_a_visited_zoom_reuses_its_cached_tiles", "crates/ui-egui/tests/view.rs::zooming_a_tiled_page_shows_its_earlier_tiles_until_new_ones_arrive"]
+notes = "capped at 8192 px per side (M3.3); tiles of a zoom already rendered stay cached while their page is on screen, so zooming away and back shows the area sharp instead of re-rendering it tile by tile (#2196)"
 
 [[feature]]
 id = "view.layout-continuous"