Show mechanism graphs as diagrams alongside evidence tables

[realmarcin]

Summary

Record mechanism sections currently show edge tables without a graph visualization. Add build-time SVG with labeled nodes, directed arrows, predicates, hover/focus details and fit/full-size controls. Keep the evidence tables below each diagram.

Fix the graph template and dynamically loaded taxon fragments. Current committed taxon records have no causal_graphs, so no published record payload changes; a representative fragment test exercises the new diagram.

The renderer uses no external graph library or browser JavaScript. It retains isolated nodes, cycles, parallel edges and explicit placeholders for unresolved endpoints. Corpus records and scientific claims are unchanged.

Validation

• Focused SVG tests cover graph integrity, escaping, keyboard access and independent markers for multiple graphs.
• Taxon payload, pagination, ATB and StrainInfo rendering tests pass (43 tests including SVG tests).
• Changed Python files pass Ruff; git diff checks pass.

Part of the fleet graph visualization audit: existing TraitMech causal diagrams, CommunityMech interaction networks and the other existing corpus maps were verified in Chromium.

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