Build the DoE region reader + automated R/LaTeX report pipeline

Closes the "still open" reader-tool note from §XXXV.10: scripts/
extract_doe_region.py spools DOE-PERSIST-TRIAL's on-disk binary ring off
disk/artemis.img into a CSV -- direct file read by default, or via a real
loop device (--loop, losetup) per the literal request. Independent
Python reimplementation of doe_region.c's own ring-walk, verified
against a live 3-record write/read round trip.

analyse_multiuser_doe.R (house style, ggplot2/svglite) and
multiuser_doe_report.tex (same preamble convention as
bare_metal_doe_report.tex) auto-discover whatever data exists and
degrade gracefully to a placeholder when a campaign has 0 trials so far
-- the whole point being this can be rebuilt at any point during a
multi-day campaign, including mid-run. build_multiuser_doe_report.sh
chains extraction -> R -> svg_to_pdf.py -> pdflatex into one command.

Found and fixed a real bug while verifying: the R script's SCRIPT_DIR
detection silently wrote charts/tables/report output to the repo root
instead of the analysis directory when invoked from a different cwd --
switched to the robust commandArgs() "--file=" idiom. Verified live both
branches (0 trials and a real 3-record write), reverted synthetic test
data and incidental svg_to_pdf.py churn on ~85 unrelated committed
figures before committing. FABRIC-3.md §XXXV.11 has the full account.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWpNjdwPtFLuVLaAq44L9K

experiments/bare_metal/analysis/report/sections/multiuser_summary_autogen.tex

@@ -0,0 +1,4 @@
+% Auto-generated by analyse_multiuser_doe.R -- do not edit by hand.
+No trial data has been persisted to the DoE region yet. This section
+will populate automatically the next time this report is built after
+the campaign has produced at least one completed trial.