test(memory): pin that the WRITE path marks stale, not the test (C4 follow-up)

C4j asserted that a stale-marked centroid falls back to uncorrected vectors — but it
called `store.markCentroidStale(fp)` **itself** to set up the state. That proves the
recall side honours the flag; it says nothing about whether production ever sets it.

Measured: deleting the one line in `#ensureCentroid` that persists the verdict

    if (status?.stale === true) this.store.markCentroidStale(fingerprint)

left **all 306 criteria green**. The wiring had no guard at all.

This is the fifth instance of one shape in this branch:

    W8   saveSpaceCentroid defined, zero production callers
    C4d  centroid built by the test's own rebuildCentroid call
    P1   vectorScores hand-built in the fixture
    D2   counters computed, never printed
    C4j  stale flag set by the criterion, not by the write path

A hand-fed input cannot prove a production path. So C4k drives only the production
entry points — `apply()` + `commit()` — and marks nothing itself:

    after a settled rebuild   stats.stale === undefined
    after 3 more commits      stats.stale === 1     (drift 3/8 = 37.5% > 20%)

Mutation — deleting that line in `#ensureCentroid`:

    not ok 12 - C4k: the write path itself marks a drifted centroid stale
    error: C4k: the WRITE path must persist the stale verdict ... undefined !== 1

## Why the drift is generated rather than forced

3 commits on top of a settled 8 puts the delta at 37.5%, past the 20% ratio — the
same path an ordinary growing space takes. Forcing `markCentroidStale` directly is
exactly what C4j does and exactly what this criterion exists to complement.

Gates: npm test 307/307 (0 skipped), npm run check 12/12, frozen faces untouched.

test/phase-b-c4-centering.test.js

@@ -684,3 +684,68 @@ test('C4j: a centroid marked stale falls back to uncorrected vectors', async (t)
     store.close()
   }
 })
+
+// ── C4k: 落标记这件事必须是写入侧自己干的，不是测试替它干 ───────────────────
+//
+// C4j 里有一行 `store.markCentroidStale(fp)` —— 那是判据**手工**打的标记，它只
+// 证明「召回侧会尊重标记」，不证明「生产路径会打这个标记」。
+//
+// 实测：把 `lib/index.js` 里那行 `if (status?.stale === true) this.store
+// .markCentroidStale(...)` 整个删掉，**全套 306 条判据全绿**。也就是说这条接线
+// 此前没有任何判据守着 —— 与 W8（saveSpaceCentroid 零生产调用点）、C4d（中心
+// 靠测试手工 rebuildCentroid）是同一个病：手工喂进去的输入，证明不了生产路径。
+//
+// 这条判据只用生产入口：`apply()` + `commit()`，自己不打任何标记。
+test('C4k: the write path itself marks a drifted centroid stale', async () => {
+  const { Context } = await import('@deepseek-ai/cordis')
+  const { SystemPrompt } = await import('@deepseek-ai/dsh-system-prompt')
+  const { ToolRuntime } = await import('@deepseek-ai/dsh-tools')
+  const { apply: applyMemory } = await import('../lib/index.js')
+  const { mkdtempSync, rmSync } = await import('node:fs')
+  const { tmpdir } = await import('node:os')
+  const { join } = await import('node:path')
+
+  const DIM = 4
+  const dir = mkdtempSync(join(tmpdir(), 'c4k-'))
+  const ctx = new Context()
+  new SystemPrompt(ctx, {})
+  new ToolRuntime(ctx, {})
+  try {
+    const mem = applyMemory(ctx, {
+      path: join(dir, 'm.db'),
+      embedder: {
+        async embedText() { return unit([1, 0.1, 0, 0]) },
+        info: () => ({ fingerprint: 'c4k:write-marks', dimension: DIM }),
+      },
+    })
+    const fp = mem.space().fingerprint
+
+    // Build a centroid, then let the scheduled rebuild land so the count is settled.
+    for (let i = 0; i < 8; i += 1) await mem.commit({ text: `块 ${i}` })
+    await new Promise((resolve) => setTimeout(resolve, 3000))
+    const settled = mem.store.loadCentroid(fp)
+    assert.ok(settled !== undefined, 'C4k precondition: a centroid must exist')
+    assert.equal(
+      settled.stats?.stale,
+      undefined,
+      'C4k precondition: a freshly rebuilt centroid carries no stale flag',
+    )
+
+    // Drift it past the 20% ratio through the production write path only.
+    for (let i = 8; i < 11; i += 1) await mem.commit({ text: `块 ${i}` })
+
+    const drifted = mem.store.loadCentroid(fp)
+    assert.equal(
+      drifted.stats?.stale,
+      1,
+      'C4k: the WRITE path must persist the stale verdict — the recall side is not '
+        + 'allowed to judge it (C3-k), so if this flag is missing, recall keeps '
+        + 'centring with a mean that no longer describes the space. Deleting that one '
+        + 'line in #ensureCentroid left all 306 criteria green, which is why this '
+        + 'criterion exists.',
+    )
+  } finally {
+    await ctx.fiber.dispose()
+    rmSync(dir, { recursive: true, force: true })
+  }
+})