rendering arc: the imp node becomes the argument's geometry, not a fake proposition

Owner-approved brief (orchestrator): the canvas stops drawing implication
nodes as first-class nodes and starts drawing them as what they are — the
point where co-premise lines converge.

- deriveView (core/graph.ts): single-premise chains collapse to ONE direct
  premise->conclusion arrow — no imp node minted (a chain is always N-in/
  1-out; one premise is the degenerate case). Multi-premise chains keep
  the imp node + fan. Dangling guard unchanged.
- physics (core/layout.ts createGraphSimulation): per-link rest length —
  a link touching an arg_-prefixed endpoint gets HALF the base link
  distance, default or user-tuned alike; everything else about imp-node
  physics citizenship unchanged.
- canvas (components/graph-canvas.ts): imp nodes are PURE GEOMETRY —
  invisible (no disc, no label) and unpickable (excluded from pickAt's
  hit-test projection). Edges INTO an imp node: headless, shaft runs to
  its CENTER (toRadius 0, no head triangle). Edges OUT: shaft starts at
  the center (fromRadius 0), arrowhead lands on the conclusion's disc
  edge. Single-premise direct arrows render as today's plain arrows.
- comment repairs (same commit): the two stale double-shaft claims
  (CanvasEdge doc + the draw-pass block) rewritten to the truth — plain
  single shafts; bundles are headless converging lines + one arrowed
  out-edge; no double-shaft glyph exists anywhere.
- tests (+9): deriveView pins (single-premise collapse, multi-premise
  fan, shared-conclusion distinctness, dangling drop); imp-link half-
  distance settle pins; arrowGeometry zero-radius laws + isImpNodeId
  pin; graph.test.ts's snapshot invariant now counts only MULTI-premise
  live chains (its fixture's single-premise chains mint zero imp nodes);
  layout settle fixture updated for the collapse.

tests/graph-canvas.test.ts

@@ -14,6 +14,7 @@ import { describe, it, it as test } from "node:test";
 import {
 	applyClusterLabels,
 	arrowGeometry,
+	isImpNodeId,
 	boxBlurRound,
 	fitTransform,
 	labelAlpha,
@@ -148,6 +149,26 @@ describe("arrowGeometry", () => {
 	test("null for coincident endpoints (never a NaN arrow)", () => {
 		assert.equal(arrowGeometry({ x: 5, y: 5 }, { x: 5, y: 5 }, 6, 6, 5), null);
 	});
+
+	// The rendering arc's geometry laws (owner, 2026-09-30): an edge INTO
+	// an imp point is drawn with toRadius 0 (headless, terminates at the
+	// center); an edge OUT of one with fromRadius 0 (starts at the center).
+	test("toRadius 0 — the tip lands exactly at the target CENTER (headless convergence)", () => {
+		const g = arrowGeometry({ x: 0, y: 0 }, { x: 100, y: 0 }, 6, 0, 5)!;
+		assert.ok(Math.abs(g.tip.x - 100) < 1e-9 && Math.abs(g.tip.y) < 1e-9, "tip = the center, past which no shaft draws");
+	});
+
+	test("fromRadius 0 — the shaft starts exactly at the source CENTER", () => {
+		const g = arrowGeometry({ x: 0, y: 0 }, { x: 100, y: 0 }, 0, 6, 5)!;
+		assert.ok(Math.abs(g.shaftFrom.x) < 1e-9 && Math.abs(g.shaftFrom.y) < 1e-9, "shaftFrom = the center");
+	});
+});
+
+describe("isImpNodeId (the pure-geometry law, rendering arc 2026-09-30)", () => {
+	test("arg_-prefixed ids are geometry; proposition ids are citizens", () => {
+		assert.equal(isImpNodeId("arg_123af5a046ba"), true);
+		assert.equal(isImpNodeId("n_abc123"), false);
+	});
 });