fix(constellarium): round 3.1 — pan restored, phantom pin removed, WebGL guard

[vicquick]

Closes the "still doesn't behave like Obsidian" thread with empirical verification: a raw-CDP behaviour suite (no playwright — playwright-python retained every exceptionThrown CDP event from a 60fps render-error storm, leaked ~3MB/s and was cgroup-OOM-killed at 4GB) runs nine asserts against the live view, all green:

assert	result
hover engages after settle	✅
hover releases on empty sky	✅
node drag runs	✅
background pan not hijacked after drag	✅
pan moves the world (node reappears at old + Δ)	✅
click pins folio	✅
pin dismisses via ×	✅
zoom is cursor-anchored	✅
zoom survives without crash	✅

Three real defects the harness surfaced:

1. Grab radius wider than hover radius — grabSubject used nodeRadius+4 with a 10-world-unit floor while the pixi hitArea is nodeRadius+2. An empty-sky press silently grabbed an unseen star; d3-drag claimed the gesture and the pan never reached d3-zoom. Grab now mirrors the hitArea: if it isn't hoverable, it isn't grabbable.
2. Phantom pin after every drag — pixi pointertap fires when down+up land on the same object, and a dragged star travels with the cursor, so every drag ended pinned. Click-to-pin now lives solely in d3-drag's end handler (pointer distance < 5 px).
3. Canvas-fallback render storm — pixi's canvas renderer has no pipe for the round-3 GL link mesh and threw on every render: frozen world, stale worldTransforms, dead hit-tests, and an error storm. Non-WebGL browsers now get a quiet notice instead of the storm.

Round-3 render core (single GPU mesh for all links — no per-frame tessellation of 58k strokes; one shared hover animation instead of 60k tween allocations) rides along with its deps: pixi.js 8.20, d3 modules, tween.js.

Dev-only test seam: window.__app exposed under import.meta.env.DEV for the behaviour suite.

🤖 Generated with Claude Code

https://claude.ai/code/session_0137xgmfhc8co2pjNDuZw6FR