confidence slider UI round: global ←/→, fraction positions, color ramp; plan-flank phantom fix

Owner UI round 2026-10-01, three rulings on the slider plus one en-route
bug:

- GLOBAL ←/→, the ↑/↓ pattern: confidence adjusts wherever focus sits
  (window keydown, typing-guarded), no click-in, no focus ring. The
  slider itself takes NO focus — tabIndex -1, mousedown prevented,
  removed from the Tab chain (rows → thinking directly again).
- Position = fraction: the thumb's scale is 0-100 with the rendered
  value the stop's own percent, so 50 sits halfway; onChange snaps to
  the nearest stop (magnet feel while dragged).
- Six-step ramp (owner pick over three bands): 0 red, 30 orange,
  50 yellow (the coin flip), 70 lime, 90 green, 100 emerald — fill AND
  handle wear the stop's color; taller 10px pill bar, custom track/thumb
  (webkit + moz), muted remainder via --muted.
- Plan-flank phantom: gating the marginalia button on a JSX element
  (always truthy) mounted an invisible clickable button in the
  bottom-right whenever a question was pending with no plan.
  planExists() now gates it (pinned in plan-window tests).

tests/plan-window.test.tsx

@@ -1,7 +1,7 @@
 import { renderToStaticMarkup } from "react-dom/server";
 import { describe, it } from "node:test";
 import assert from "node:assert/strict";
-import { PlanMarginalia } from "../components/plan-window.tsx";
+import { PlanMarginalia, planExists } from "../components/plan-window.tsx";
 import { blockLayout, type PopupMessage } from "../components/plan-block.tsx";
 import type { PlanTree } from "../src/core/contracts.ts";
 
@@ -44,6 +44,19 @@ function transcript(tree: PlanTree, taggedNodeId?: string): PopupMessage[] {
 	];
 }
 
+describe("planExists", () => {
+	it("false without a tree — the phantom-click gate (owner report, 2026-10-01)", () => {
+		// A JSX element is always truthy; the WRAPPER must gate on real
+		// presence or an invisible clickable button sits in the flank.
+		assert.equal(planExists([]), false);
+		assert.equal(planExists(transcript({ nodes: [], edges: [], version: 0 })), false);
+	});
+
+	it("true once the transcript holds a plan tree", () => {
+		assert.equal(planExists(transcript(CHAIN)), true);
+	});
+});
+
 describe("PlanMarginalia render", () => {
 	it("renders nothing until a tree exists", () => {
 		assert.equal(renderToStaticMarkup(<PlanMarginalia messages={[]} />), "");