Prober: no claims under PROBE_MIN_FREE_GB free on the NVMe

At ~200 articles/s what the prober stores (inspections, par2 file lists,
the WAL) took ~0.8-1.1 GB an hour of homelab's root, and nothing stopped it
at the 10 GB floor. Below 12 GB free it claims nothing (resumed 1 GB above):
the lanes go dry and its share shrinks to its minimum.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/disk_space.spec.ts

@@ -0,0 +1,22 @@
+import { test } from '@japa/runner'
+import { DiskSpace } from '#services/disk_space'
+
+test.group('DiskSpace', () => {
+  test('short below the floor, fine again a margin above it; off without a floor', async ({ assert }) => {
+    let gb = 15
+    const space = new DiskSpace('probing', '/', 12, async () => gb * 1e9, 0)
+    assert.isFalse(await space.short())
+    gb = 11.5
+    assert.isTrue(await space.short())
+    // Back over the floor, not yet over the margin: still short.
+    gb = 12.5
+    assert.isTrue(await space.short())
+    gb = 13.1
+    assert.isFalse(await space.short())
+    assert.isFalse(await new DiskSpace('probing', '/', 0, async () => 0, 0).short())
+  })
+
+  test('measures a real filesystem', async ({ assert }) => {
+    assert.isFalse(await new DiskSpace('probing', '/', 0.001).short())
+  })
+})