test(subprocess): restore host-exit readiness handshake

packages/subprocess/subprocess-local/tests/fixtures/process-exit-host.ts

@@ -13,6 +13,7 @@ if ((kind !== 'ordinary' && kind !== 'terminal')
 }
 
 const treeState = join(root, 'tree.json')
+const ready = join(root, 'ready')
 const proceed = join(root, 'proceed')
 const managedTree = fileURLToPath(new URL('./managed-tree.ts', import.meta.url))
 
@@ -57,6 +58,7 @@ const published = JSON.parse(await readFile(treeState, 'utf8')) as { root?: unkn
 if (!Number.isSafeInteger(published.root) || !Number.isSafeInteger(published.descendant)) {
   throw new Error('managed tree published invalid process ids')
 }
+await writeFile(ready, 'ready')
 await waitForFile(proceed)
 
 if (trigger === 'dispose') {

packages/subprocess/subprocess-local/tests/process-exit.spec.ts

@@ -107,6 +107,10 @@ async function runScenario(kind: ManagedKind, trigger: ExitTrigger) {
   let treeGone = false
   try {
     state = await readTree(join(root, 'tree.json'))
+    await vi.waitFor(() => readFile(join(root, 'ready'), 'utf8'), {
+      interval: 10,
+      timeout: scenarioTimeoutMs,
+    })
     if (process.platform !== 'win32') identities = await captureIdentities(createProcessInspector(), state)
     await writeFile(join(root, 'proceed'), 'proceed')
     const outcome = await child