Spread fleet replicas across extra machines and show message RTT percentiles

machines.json plus Add-machine UI place copies on the least-loaded host.
Monitor samples processing RTT and displays min, avg, p50, and p90 for
capacity planning. Remote hosts run src/agent.js.

packages/verae-fleet/test/rtt.test.js

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+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { summarizeRtt, RttWindow } from '../src/rtt.js';
+import { pickMachine } from '../src/machines.js';
+
+describe('RTT planning stats', () => {
+  it('computes min, avg, p50, p90 in order', () => {
+    const s = summarizeRtt([10, 20, 30, 40, 50, 60, 70, 80, 90, 100]);
+    assert.equal(s.count, 10);
+    assert.equal(s.minMs, 10);
+    assert.equal(s.avgMs, 55);
+    assert.ok(s.p50Ms >= s.minMs && s.p50Ms <= s.p90Ms);
+    assert.ok(s.p90Ms <= 100);
+  });
+
+  it('empty window is nulls', () => {
+    const s = new RttWindow().stats();
+    assert.equal(s.count, 0);
+    assert.equal(s.minMs, null);
+  });
+});
+
+describe('machine placement', () => {
+  it('spreads onto the least-loaded eligible host', () => {
+    const machines = [
+      { id: 'a', enabled: true, capacity: 4, roles: ['*'] },
+      { id: 'b', enabled: true, capacity: 4, roles: ['*'] },
+    ];
+    const instances = [{ machine: 'a', pid: 1 }];
+    const pick = pickMachine(machines, instances, 'tree-node', 'tree-node');
+    assert.equal(pick.id, 'b');
+  });
+
+  it('skips machines that do not allow the role', () => {
+    const machines = [
+      { id: 'edge', enabled: true, capacity: 8, roles: ['zappier-edge'] },
+      { id: 'trees', enabled: true, capacity: 8, roles: ['tree-node'] },
+    ];
+    const pick = pickMachine(machines, [], 'tree-node', 'tree-node');
+    assert.equal(pick.id, 'trees');
+  });
+});