fix(subprocess): consumers with one mixed env map split DSH_* onto the managed channel

Codex review of the acp-only fix found lsp-local carries the identical
defect: its server config's unrestricted env merges into the connection's
ordinary spawn channel, so a configured DSH_* fact crashed the spawn with
the reserved-namespace rejection. The partition now lives on the seam as
splitEnvChannels() beside the scrub it complements; the ACP run and the
LSP connection both use it, and each proves child delivery end-to-end
(MOCK_ECHO_ENV / LSP_FAKE_ECHO_ENV fixture knobs). Seam + consumer README
rows updated (en+zh, re-recorded). bash-local is already two-channel;
mcp/pty/sdk bypass the seam and only share the scrub.

packages/lsp/lsp-local/tests/connection.spec.ts

@@ -51,6 +51,15 @@ describe('LspConnection', () => {
     expect(conn.pid).toBeGreaterThan(0)
   })
 
+  it('routes explicit DSH_* env entries onto the managed channel', async () => {
+    // A configured DSH_* fact must reach the child: the ordinary channel
+    // rejects the reserved namespace, so the connection's spawn must split it
+    // onto dshEnv. The fixture echoes the named variable back as hover text.
+    const conn = connect({ LSP_FAKE_ECHO_ENV: 'DSH_LSP_TEST_FACT', DSH_LSP_TEST_FACT: 'managed' })
+    await conn.request('initialize', { capabilities: {} })
+    expect(await conn.request('textDocument/hover', {})).toEqual({ contents: 'managed' })
+  })
+
   it('rejects a request when the server replies with an error', async () => {
     const conn = connect({ LSP_FAKE_ERROR: '1' })
     await conn.request('initialize', { capabilities: {} })