MetricsCollector writes one row per epoch to <run_dir>/metrics.csv, but
nothing read or plotted it. giant/training/plots.py reads the CSV header
dynamically (the column set varies by run: flow/ddpm vs wgan, routed vs
not) and renders loss/lr/accuracy/grad-norm/router/wgan-balance/throughput
plots with the same plotstyle conventions giant/analysis/render.py uses,
skipping any figure whose columns aren't present for a given run.
Wired up as `giant analyze metrics <run_dir>`, writing PDFs into the same
gitignored analysis_runs/ directory `analyze prep`/`submit` already use
(derive_metrics_dir mirrors derive_run_dir) rather than into the training
run directory itself.
