report.py mixed DB-reading generation with string formatting: the four
render_*_report(run_id, session) functions each re-fetched via
generate_report, so the HTML/Markdown/JSON formatting was welded to storage
and could not be unit-tested from a plain dict. Extract the formatting into a
new pure report_render module whose renderers take the already-built report
dict (no session, no storage import). Migrate every caller to generate-then-
render, delete the old coupled renderers with no back-compat shim, and drop
the _aggregate_runs middle-man alias in favour of metrics.aggregate_runs.
