Replace ASCII architecture diagrams with colored Mermaid diagrams

[rustnew]

What changed and why

Follow-up to #4 (merged before this change landed -- same content,
rebased onto current main). The 14 ASCII-art diagrams in docs.md
(§8, §9.5, §9.9-9.10, §9.15, §12-14, §17, §19.1, §25 x3, §29) render as
literal monospace text and don't scale to a public launch page. Replaced
with equivalent Mermaid flowcharts (a quadrant chart for §9.9's Pareto
front), colored via per-diagram classDef (soft red = reject/stop, green
= confirm/ground-truth, blue = terminal "next generation" states).

Evidence

• N/A (presentation-only PR) -- content unchanged, every diagram
  carries the exact same nodes/flow as its ASCII original.
• No claim differs: nothing here changes what any diagram says, only
  how it's drawn.
• N/A -- no negative result introduced here.

Colors are defined per-diagram via classDef (not page-level CSS), so
they render correctly and consistently both on the built GitHub Pages
site and natively when viewing docs.md directly on GitHub. Plain
example-output/list blocks that aren't actually diagrams (learning-rate
output example, failure-category list, TRAIN/VAL/TEST description,
baseline comparison list) are intentionally left as plain code blocks.

docs/index.html regenerated from docs.md via the existing build
script, extended to convert Python-Markdown's fenced-code Mermaid output
into the <pre class="mermaid"> shape mermaid.js expects, plus a
page-level mermaid.initialize() call (theme matched to the site's
palette) loaded from jsdelivr.

Checklist

• CI (reproduce.yml) passes on this branch (docs-only change).
• No claim in this PR is asserted without a script/report backing it.
• data/meta_dataset.db not touched.
• README/docs.md updated if this PR changes a number or conclusion
  referenced there -- N/A, no numbers or conclusions changed.

Open questions / requested feedback

None -- straightforward presentation change. Flagging only that this
branch was rebased onto current main because #4 merged mid-flight;
the diff here is identical in substance to what was originally pushed to
docs/full-spec-page.

[coderabbitai]

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