feat: add site-sonar page — submarine radar content discovery

[arc-butler]

What

A retro submarine-style radar/sonar screen that sweeps through all site content (posts, projects, books) as glowing blips on a rotating radar display.

How it works

• Positioning: Each piece of content is a blip on the radar. Its angle is deterministically derived from the title, and its distance from center reflects recency (newer = closer).
• Sweep: A rotating radar beam sweeps clockwise, illuminating blips as it passes over them. Blips fade after ~5 seconds, then reappear on the next pass — just like real maritime radar.
• Interaction: Hover reveals the title and type, click navigates to the item.
• Zero dependencies: Pure vanilla Canvas JS. No libraries, no API keys.
• Data: Injected via Jekyll Liquid into window.__sonarData, so the radar is always up to date with site content.
• Theme-aware: Tooltip and legend use theme CSS variables.

Files changed

• _pages/sonar.md — New page with frontmatter, embedded CSS, Liquid data injection, and JS include
• assets/js/site-sonar.js — Vanilla JS sonar visualization (~480 lines)

Why

Follows the existing site pattern of useful data presented through an absurd lens: Travel Map (geographic map → Leaflet), Vital Signs (site analytics → hospital monitor), Game of Life (site data → cellular automata), Site Sonar (content discovery → submarine radar).

Weird: WW2 submarine radar on an academic homepage.
Sensible: Visual content discovery organized by recency and type.