docs: replace ASCII diagrams with Mermaid visuals + brand CSS

- Add sphinxcontrib-mermaid>=0.9.2 to requirements.txt
- Add custom.css with orange brand theme (admonitions, tables, mermaid containers)
- Replace Complete Algorithm ASCII pseudocode with Mermaid flowchart (differential_evolution.md)
- Add Mermaid stateDiagram-v2 for Bull/Normal/Bear regime transitions (hmm.md)
- Replace Rust module file tree with Mermaid graph TD (point_processes.md)
- Configure mermaid_version=10.9.0 and dark+orange themeVariables in conf.py

docs/source/conf.py

@@ -20,6 +20,7 @@ extensions = [
     'sphinx.ext.intersphinx',
     'sphinx.ext.mathjax',
     'myst_parser',
+    'sphinxcontrib.mermaid',
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -76,3 +77,10 @@ myst_enable_extensions = [
     "deflist",
     "dollarmath",
 ]
+
+# Custom CSS
+html_css_files = ["custom.css"]
+
+# Mermaid configuration
+mermaid_version = "10.9.0"
+mermaid_init_js = "mermaid.initialize({startOnLoad:true, theme:'dark', themeVariables:{primaryColor:'#f97316',primaryTextColor:'#fff',primaryBorderColor:'#ea6a0a',lineColor:'#fb923c',secondaryColor:'#1e293b',tertiaryColor:'#0f172a'}});"