docs(adr-0021): analytics — one semantic dataset layer, report/dashboard as pure presentation (#1415)

Redesign the analytics surface from a clean sheet: introduce a single
semantic `dataset` layer (a thin wrapper over the existing QuerySchema
engine, declaring named dimensions + measures), and collapse `report`
(pivot) and `dashboard` (canvas) into pure presentations that bind to a
dataset by reference and select dimensions/measures by name.

Kills the three inline single-object query grammars (Report.columns/
groupings, Widget.category/value/aggregate, ListChartConfig), unlocks
joins everywhere, gives one source of truth per metric, and keeps
Airtable-style ergonomics via inline-dataset desugaring.

Records the dashboard→dataset (not dashboard→report) rationale and a
report-embed widget for table-in-dashboard composition. Big-bang
migration, no back-compat; one-time codemod.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>

Author: xuyushun441-sys