Renderer swap: drop sigma.js for a hand-rolled single-canvas 2D renderer

Implements Option B from docs/research/graph-rendering.md. sigma.js v3
(WebGL) is replaced by one canvas, one rAF loop with a dirty flag, and
d3-zoom for the camera — eliminating the entire bug class from the
sigma-era bolt-on architecture:

- Hull/node desync during pan (two independent render loops) is now
  structurally impossible: hull fills, dashed hull strokes, edges with
  arrowheads, node discs, region labels, node labels, and hover glow
  are all drawn in ONE frame under ONE camera.
- The settings-change canvas-sizing race (everything shifting up-left)
  is gone: settings changes only redraw; the canvas and camera persist
  (verified live: camera identical to 15 decimals across a slider drag).
- The GPU floor is gone: idle schedules NO frames at all (verified via
  pixel-identity over 600ms idle); pan redraws exactly once per frame.

components/graph-canvas.ts: GraphCanvasRenderer class plus pure helpers
(zoomRatioOf, labelAlpha, fitTransform, arrowGeometry, pickNearest).
The sigma camera-ratio convention (1 = fit, smaller = zoomed in) is kept
verbatim so persisted labelZoom settings survive the swap; node labels
and cluster region labels cross-fade over the same LABELS_FADE_SPAN
(0.5, unchanged). Picking is a nearest-node distance check with the
10px pad that replaces the old padded-picking vertex shader. Hull math
stays in components/cluster-geometry.ts, reused as-is. No ctx.filter,
ever.

graph-view.tsx keeps every feature: fetch/refetch on runs-empty, FA2
run-then-freeze layout, localStorage render settings + sanitizers,
search filtering with edge drops and card-close, detail card with
flipped anchoring and per-source provenance, settings panel, import
button, pipeline status chips. Only the renderer plumbing changed.

tests/graph-canvas.test.ts pins the pure math: ratio direction, label
gate + inverse complement (alphas sum to 1), fit transform padding,
arrowhead tip-on-disc-edge and symmetry, picking pad/ties/null cases.

Live-verified end-to-end on the real 338-node graph (dev server
port 3002): single canvas, paint at fit, hover cursor+glow+fade, click
card open/anchored/close, stage click, search filter to zero with hint,
label gate opens via slider and zoom, cluster-labels toggle, zoom-to-
cursor 0.56px anchor drift, camera persistence across settings, zero
idle repaints. sigma dependency removed from package.json; d3-zoom and
d3-selection added.

Note: the settings feed effect needed isEmpty in its deps — without it
the renderer mounted with settings=null and every draw early-returned
(blank canvas); caught live, not by typecheck.

package-lock.json

@@ -24,6 +24,8 @@
 				"class-variance-authority": "^0.7.1",
 				"clsx": "^2.1.1",
 				"cmdk": "^1.1.1",
+				"d3-selection": "^3.0.0",
+				"d3-zoom": "^3.0.0",
 				"embla-carousel-react": "^8.6.0",
 				"graphology": "^0.26.0",
 				"graphology-communities-louvain": "^2.0.2",
@@ -43,7 +45,6 @@
 				"react-dom": "^19.2.8",
 				"react-jsx-parser": "^2.4.1",
 				"remark-breaks": "^4.0.0",
-				"sigma": "^3.0.3",
 				"streamdown": "^2.5.0",
 				"tailwind-merge": "^3.6.0",
 				"tokenlens": "^1.3.1",
@@ -13342,16 +13343,6 @@
 				"url": "https://github.com/sponsors/ljharb"
 			}
 		},
-		"node_modules/sigma": {
-			"version": "3.0.3",
-			"resolved": "https://registry.npmjs.org/sigma/-/sigma-3.0.3.tgz",
-			"integrity": "sha512-5H0zFlx6/NTQpqBg4Rm569ZOpnBOXMaS25UQThIWMU3XyzI5AhmorK/gnl87BvJBLhQd0tW4C0LIp3enWzMoNw==",
-			"license": "MIT",
-			"dependencies": {
-				"events": "^3.3.0",
-				"graphology-utils": "^2.5.2"
-			}
-		},
 		"node_modules/source-map-js": {
 			"version": "1.2.1",
 			"resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",