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

Adds same-trace baseline diffing to the Heap Dump Explorer flamegraph and
colours it pprof-style: each node's fill is keyed on the signed, normalised
change vs the baseline (green = smaller, grey = unchanged, red = larger;
intensity by magnitude). Width can be |Δ| or current value; colour basis is
absolute (Δ / max|Δ|) or relative (Δ / baseline, clamped).

Same-trace path pairs by name-based path hash via _graph_scan!, with the
intermediate CTEs forced through SQLite as MATERIALIZED so the final
LEFT JOIN uses an automatic index (drops the cost from ~157 s to ~8 s
natively on a real ~110 k-node system_server class tree).

Cross-trace path (different baseline trace, different engine) pairs in JS
by stable name-path and writes the paired rows back into the current
engine as a TEMP TABLE the flamegraph SQL SELECTs from. The downstream
value / colour-hint SQL is identical to the same-trace path.

The diff legend (bottom-right of the flamegraph) paints a gradient from
the same colour function the node fills use, with a standard Perfetto
outlined Button label indicating the colour basis (absolute / relative).