docs: reframe design, report, and slides for the general DataFusion community

Strip PR #11 / round-6 / stacked-branch chrome from design.md and
report.md; frame everything as a proposed change on top of
apache/datafusion@main. Refresh report.md per-query tables with
current data from MAIN-{no,}pushdown vs R6-STACK-pushdown so the
drill-downs match the headline numbers (TPC-H flips from regression
to win). Tighten the slide speaker notes accordingly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb