Add diagram support

[natolambert]

Summary

Add support for rendering diagrams from text definitions inside fenced code blocks, complementing the existing chart blocks (which handle data visualization).

Motivation

Research talks frequently need structural/flow diagrams — model architectures, training pipelines, system diagrams, timelines. Currently these require external image files. Text-based diagrams would be git-friendly, agent-drivable, and consistent with Colloquium's markdown-native philosophy.

Proposed syntax

graph LR
    A[Training Data] --> B[SFT]
    B --> C[Reward Model]
    C --> D[RLHF]
    D --> E[Aligned Model]

Loading

Options

• Mermaid.js — most popular, broad diagram type support (flowcharts, sequence, Gantt, state, mindmap, quadrant). Single JS file, CDN-loadable. Already used by GitHub markdown rendering.
• Other libraries to evaluate: D2, Pikchr

Use cases for research talks

• Model architecture flowcharts (data → SFT → RM → PPO)
• System/infrastructure diagrams
• Gantt-style research timelines
• Sequence diagrams (multi-agent interactions, API flows)

Implementation notes

• Follows the existing pattern: CDN script (like KaTeX, highlight.js), fenced code block handler in build.py
• No new Python dependencies needed
• Diagrams render as inline SVG in the self-contained HTML output
• Charts = data visualization (bar, line, scatter); Diagrams = structural/flow — no overlap