feat(explorer): WASD keyboard navigation; 3D lines show through dimmed terms

[CMaintz]

Owner requests: 3D links vanishing behind dimmed terms, and WASD keyboard navigation in 2D and 3D. Decision note: A97 (A96 is taken by open PR #64).

3D: lines behind receded terms

All spheres are transparent (nodeOpacity 0.95), so three.js sorted each one against the single merged web by distance. A receded (dimmed) sphere that happened to draw first wrote depth and erased every line behind it. Only comets and glow showed through.

The fix is a fixed renderOrder: glow, then solid spheres (these still write depth and occlude), then lines (the web plus the focused link objects), then receded spheres (depthWrite=false below EXPLORER.three.solidOpacity), then comets. The sphere materials belong to 3d-force-graph and it swaps them on its own schedule, so a scene.onBeforeRender pass applies the order every frame.

Verified with headless screenshots: with a term selected and the camera zoomed in on receded spheres, the lines now pass through them. Before the fix they stopped at the sphere's edge.

Keyboard navigation (explorer-keys.ts + tests)

• 2D: W A S D and the arrows pan. Q/E and -/+ zoom about the centre of the clear area.
• 3D: W/S move forward and back, closing in on the orbit centre and pushing it ahead once near. A/D move sideways. Q/E move down and up (Space also moves up). The arrows orbit.
• Shift triples the speed in both views.
• The OrbitControls target stays in sync, so mouse orbit still works afterwards.
• Movement is velocity-based with an exponential ease, so it doesn't depend on frame rate. Under reduced motion there is no easing.
• Letters and arrows are read by physical key. + and - are read by character, so they work on a Danish layout.
• Scope: keys only act when focus is inside the map host, or when nothing has focus and the pointer is over the map. They never fire with Alt/Ctrl/Cmd held, and never in fields, the bar, popovers or the term panel, so the panel's ←/→ still step through connections.
• Map host: tabindex=0, role="application", and an aria-label with a "use W A S D" hint. The open legend ends with a one-line key hint for the current view (EN + DA).
• Ctrl as "down" is left out: Ctrl+W closes the tab and Ctrl+drag pans.

Checks

• mise run fix && mise run gate: exit 0.
• fps (headless Edge, 2x CPU throttle), before / after:

View	Pan	Zoom	Orbit	Hover
2D	46 / 49	46 / 54	–	59 / 59
3D	–	–	49 / 60	59 / 54

The 3D hover drop is within run-to-run noise.

• Merged with origin/main (light theme feat(theme): complete light theme; cream map replaces the night map #60, fix(theme): cream only behind the Explorer and lab; About credits #66, feat(copy): free forever (home + About) #67).

Do not merge without the owner.