ui: Heap Dump Explorer flamegraph diff with pprof-style coloring

Add baseline / diff mode to the Heap Dump Explorer. Load a second trace (or
pick a second dump from the same trace) as a baseline and compare it against
the current dump across every tab:

- Overview diff, and a Flamegraph diff coloured pprof-style on the signed,
  normalised delta (green = smaller, grey = unchanged, red = larger; absolute
  or relative basis per metric) with a matching legend.
- Diff grids for Classes, Objects, Dominators, Bitmaps, Strings and Arrays,
  plus a per-object diff view.
- A top-bar RadioGroup toggles Diff / Current / Baseline.

Same-trace diffs run as a single SQL JOIN against android_heap_graph_*;
cross-trace diffs pair objects in JS across the two trace_processor engines
(NUL-free dense path ids, so the keys are portable to the native engine) and
write the paired rows back into the current engine. Pooled baseline engines
are torn down via the trace's trash on close.

Rebased onto current upstream/main, which moved the explorer onto a
store-backed session with a persisted_state permalink model after this work
was first written. The diff state is re-ported onto that model: object tabs
serialize their (currentId, baselineId) pair, the page keeps a content-keyed
tab map plus an (engine, filter)-keyed overview cache for the baseline and
current sides, and all page-level caches reset per trace load. Adds a
persisted_state round-trip unit test and fixes stale mode-toggle selectors in
the e2e tests (the widget is a RadioGroup, not SegmentedButtons).