feat(memory): give C5's reason an outlet, with a read that cannot schedule (C5 follow-up)

28866ac made `centroidStatus` name why centring is disabled. Nothing read it.
`stats()` reported `centroid` from `loadCentroid` — the raw row, which says WHAT
the centre is and not WHY centring is or is not happening — and the only two
callers of `centroidStatus` in `lib/` were `centroidRecallState` (which spreads
the result but is itself only used by tests) and `#ensureCentroid` on the write
path, which discards the return value. A grep for `.reason` under `lib/` returned
nothing but the definition itself.

Same failure as D2 one commit earlier: the value existed, was covered, and no
operator could see it.

## What this adds

`stats().centroid_state` — the verdict, alongside the existing raw `centroid`:

    centroid        the stored row (dim, mean, vectorCount, stats)
    centroid_state  { available, stale, reason, delta, rebuildScheduled, ... }
                    reason ∈ ok | no_centroid | dim_mismatch | zero_sample |
                             stale | isotropic

`centroid` keeps its exact previous shape, so nothing downstream changes.

## The part that is easy to get wrong

`centroidStatus` **registers a rebuild** when it judges a space stale. A stats
read must not mutate anything, so the call passes `schedule: () => {}` — with
the no-op scheduler `rebuildScheduled` is always false here, which is what a pure
read should report. Without it, every operator polling `stats()` on a drifted
space would keep the rebuild loop alive.

## Criterion

`test/phase-b-c5-reasons.test.js` gains a ninth case driving the real plugin:

    fresh space            -> reason 'no_centroid'
    after 8 commits + wait -> 'ok', available true, raw row and verdict both present
    drift past the ratio   -> 'stale', stale true, rebuildScheduled FALSE

The last assertion is the one that matters: it distinguishes "reports staleness"
from "acts on it".

Mutation — deleting the `centroid_state` field:

    not ok 9 - C5: stats() exposes the reason without scheduling a rebuild
    error: "Cannot read properties of undefined (reading 'reason')"

Also removed a tautology I had written in the same case
(`assert.equal(s.centroid, s.centroid, ...)`) — it can never fail, and a criterion
that cannot fail is worse than none.

Gates: npm test 300/300 (0 skipped), npm run check 12/12.

lib/index.js

@@ -836,6 +836,22 @@ export class StableMem extends Service {
       states,
       ...this.store.vectorStats(space?.fingerprint ?? ''),
       centroid: space === null ? null : (this.store.loadCentroid(space.fingerprint) ?? null),
+      // ★ C5's `reason` needs an outlet, and this is it.
+      //
+      // `centroid` above is the raw row (`loadCentroid`), which says WHAT the
+      // centre is but not WHY centring is or is not happening. `reason` is the
+      // answer to the second question — no_centroid / dim_mismatch / zero_sample /
+      // stale / isotropic / ok — and until now it was computed and thrown away:
+      // nothing under lib/ read the value `centroidStatus` returns.
+      //
+      // ★ `schedule: () => {}` is load-bearing. `centroidStatus` REGISTERS a
+      // rebuild when it judges a space stale; a stats read must not mutate
+      // anything. With the no-op scheduler, `rebuildScheduled` is always false
+      // here, which is exactly what a pure read should report.
+      centroid_state: space === null ? null : this.store.centroidStatus(space.fingerprint, {
+        queryDim: space.dimension,
+        schedule: () => {},
+      }),
       // Per-call, not library state: NULL until a recall has happened.
       last_recall: this.lastRecall,
     }

test/phase-b-c5-reasons.test.js

@@ -191,3 +191,72 @@ test('C5: centroidRecallState keeps its stale:null projection but carries the re
     store.close()
   }
 })
+
+// ── C5 的出口：reason 必须能被运维读到，而且读的时候不许有副作用 ─────────────
+//
+// 上面八条都在 store 层证明「六种原因可区分」。但区分出来没有出口，等于只做完
+// 一半：`centroid` 这一项给的是 `loadCentroid` 的原始行，它说得出「中心是什么」，
+// 说不出「为什么没在校正」。全仓曾经没有任何地方读 `centroidStatus` 的返回值。
+//
+// 现在它从 `stats().centroid_state` 出去。两条要求：
+//   1. reason 与 store 层判定一致；
+//   2. **纯读** —— `centroidStatus` 判 stale 时会登记一次重建，而 stats 是只读
+//      接口，不许因此产生写副作用。用空调度器把它按住，`rebuildScheduled`
+//      在这里恒为 false，这正是纯读该有的样子。
+test('C5: stats() exposes the reason without scheduling a rebuild', 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 unit = (v) => { const n = Math.hypot(...v); return n === 0 ? v : v.map((x) => x / n) }
+  const dir = mkdtempSync(join(tmpdir(), 'c5-stats-'))
+  const ctx = new Context()
+  new SystemPrompt(ctx, {})
+  new ToolRuntime(ctx, {})
+  try {
+    // ── fresh space: no centroid yet ──────────────────────────────────────
+    const mem = applyMemory(ctx, { path: join(dir, 'm.db'), embedder: {
+      async embedText() { return unit([1, 0.1, 0, 0]) },
+      info: () => ({ fingerprint: 'c5:stats:test', dimension: 4 }),
+    } })
+    assert.equal(
+      mem.stats().centroid_state.reason,
+      'no_centroid',
+      'C5: stats() must say WHY centring is off, not merely that no row exists',
+    )
+
+    // ── after writes: healthy ─────────────────────────────────────────────
+    for (let i = 0; i < 8; i += 1) await mem.commit({ text: `块 ${i}` })
+    await new Promise((resolve) => setTimeout(resolve, 3000)) // let the drift rebuild land
+    let s = mem.stats()
+    assert.equal(s.centroid_state.reason, 'ok', 'a built, current centroid reports ok')
+    assert.equal(s.centroid_state.available, true)
+    // Both surfaces coexist: `centroid` is the raw row, `centroid_state` is the
+    // verdict. Asserting they are the SAME object would be a tautology; the
+    // meaningful claim is that the raw row still carries what it always did
+    // (dim / mean) while the verdict carries the reason.
+    assert.equal(typeof s.centroid?.dim, 'number', 'the raw centroid row is still reported')
+    assert.ok(Array.isArray(s.centroid_state === null ? null : s.centroid?.mean), 'and its mean is intact')
+    assert.equal(typeof s.centroid_state.reason, 'string', 'the verdict names a reason')
+
+    // ── drift past 20%: stale, and stats() must NOT register a rebuild ────
+    for (let i = 8; i < 16; i += 1) await mem.commit({ text: `块 ${i}` })
+    s = mem.stats()
+    assert.equal(s.centroid_state.reason, 'stale', 'the drift is what the operator needs to see')
+    assert.equal(s.centroid_state.stale, true)
+    assert.equal(
+      s.centroid_state.rebuildScheduled,
+      false,
+      'C5: stats() is a READ. If it schedules a rebuild it has become a write path, '
+        + 'and every operator polling stats() would keep the rebuild loop alive.',
+    )
+    assert.ok(s.centroid_state.delta > 0.2, `precondition: the drift must exceed the ratio; got ${s.centroid_state.delta}`)
+  } finally {
+    await ctx.fiber.dispose()
+    rmSync(dir, { recursive: true, force: true })
+  }
+})