tp,ui: add PROFILE_FROM_TREE aggregate and expose it in flamegraphs

PROFILE_FROM_TREE(id, parent_id, frame_name, self_value, sample_type,
unit) is a new SQL aggregate that converts any tree-shaped table into
a serialized pprof Profile. The stdlib module `pprof.from_tree`
exposes it as `_pprof_from_tree!`, composing on top of the
`std.trees.*` operators so users can filter, propagate, and emit
pprof through the same primitives.

The Flamegraph widget gains a "Download as pprof" toolbar button.
Encoding lives in the engine; the widget delegates via a callback so
the bytes always match the SQL surface.

The SQL query results panel and SQL table tab gain an "Add Flamegraph"
control mirroring "Add debug track": the user picks the
id/parent_id/name/value columns of any query, optionally overrides
sample type and unit, and the result opens as an ephemeral flamegraph
tab that inherits the toolbar Download button.

Documentation under analysis/pprof-from-tree.md describes the SQL
surface and walks through recipes for heap dumps, callstack samples,
message-queue stacks, and Java heap dominator trees. Diff tests cover
the aggregate and the stdlib macro.

Change-Id: I27106469fc9c74dc45f494ed87a70c3b76f9c089