tp: add flamechart operator for sampled stack rendering

To render sampled call stacks as a flame chart over time, the UI needs
the maximal-run decomposition of a stream of (ts, leaf callsite)
samples: for each stack depth, the time ranges over which the same
frame was continuously present. Computing this in SQL needs unbounded
recursive expansion and doing it in JS means shipping every sample to
the UI, so implement it as a C++ operator mirroring the flamegraph
split: a pure sweep in flamechart.cc and a SQLite aggregate
(__intrinsic_flamechart) wrapping it.

The sweep is a single streaming pass over ts-ordered points: points
whose stack shares a prefix with the previous one extend the open
segments at those depths; only divergent depths close and open
segments. Leaf ids resolve through the id -> row index persisted on
Tree, and output rows stream straight into a dataframe.

Expose the operator to SQL as _flamechart_runs! in the std.trees
stdlib module, alongside the other tree operations.