refactor(code-runtime-python): move the package into packages/experimental

The CPython code runtime's complete public contract is experimental, so it
moves to packages/experimental per the experimental-packages rules: npm name
@deepseek-ai/dsh-experimental-code-runtime-python, private: true, no
publishConfig. All references updated (code-runtime READMEs, config-catalog
and module-graph regenerated with zh alignment, tsconfig paths, doc-standard
and workspace-constraints scripts, the fd-3 and settlement Agent Notes, and
the package README links); md-links and translation pairing pass, and the
suite still runs green.

packages/experimental/code-runtime-python/tests/residual-detach.spec.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from 'vitest'
+import { detachResidual } from '../src/index.ts'
+
+describe('detachResidual — fd-3 residual detachment', () => {
+  it('returns a copy that does NOT share the source frame allocation', () => {
+    // Simulate the data handler's state: one large joined frame from
+    // Buffer.concat, sliced past its newline to leave a small residual VIEW.
+    // The fixture MUST stay larger than Node's Buffer pool threshold
+    // (`Buffer.poolSize / 2`, 4 KiB): above it `Buffer.from` allocates a
+    // dedicated backing store whose `byteLength` equals the copy's length,
+    // which is what the byteLength assertion below pins. A smaller residual
+    // would be pooled into an 8 KiB shared ArrayBuffer, making `byteLength`
+    // report 8192 and the assertion false-fail even though the fix is intact.
+    const joined = Buffer.alloc(1024 * 1024, 0x61) // 1 MiB backing allocation
+    joined[512] = 0x0a // a newline partway through
+    const residual = joined.subarray(513) // a view onto `joined`'s backing store
+
+    // Before the fix the handler carried this view forward verbatim, pinning the
+    // whole 1 MiB `joined` allocation behind a residual that reports far fewer
+    // bytes. A right-sized copy must not point back into `joined`.
+    const [carried] = detachResidual(residual)
+
+    expect(carried).toBeDefined()
+    expect(carried!.length).toBe(residual.length)
+    expect(carried!.equals(residual)).toBe(true)
+    // The core invariant: the copy does NOT share the source frame's backing
+    // store, so retaining it cannot pin the 1 MiB allocation.
+    expect(carried!.buffer).not.toBe(joined.buffer)
+    // And the copy's own backing store is sized to its content — not the whole
+    // frame. Holds because the fixture exceeds the pool threshold (see above);
+    // a subarray view would report the source's full byteLength here.
+    expect(carried!.buffer.byteLength).toBe(carried!.length)
+  })
+
+  it('carries nothing forward for an empty residual', () => {
+    expect(detachResidual(Buffer.alloc(0))).toEqual([])
+  })
+})