fix(e2b): address review round on cadence config, disposal, and SDK edge cases

- subprocess-e2b: the 20 ms remote poll cadence becomes a validated pollMs
  Config field (each tick is one control-plane request); the README documents
  the latency-versus-request-count trade.
- subprocess-e2b: extract src/remote.ts owning asError, signalOpts,
  commandOpts, delay, waitTick, and one tolerant signalRemoteGroups shared by
  the pgid-keyed process ladder and sid-keyed terminal ladder, so the two
  teardown paths keep identical error tolerance.
- subprocess-e2b: service disposal aggregates sibling cleanup failures into
  one AggregateError instead of discarding all but the first.
- subprocess-e2b: waitForProcessGroupId refuses published group ids <= 1, so
  a same-UID rewrite of the pid file cannot aim termination at kill -- -1;
  README documents the same-UID control-state limitation.
- subprocess-e2b: drain-grace expiry now releases an inherited-output E2B
  callback blocked on host backpressure before disconnecting, so the SDK
  settlement cannot stay pinned behind an unread host stream.
- subprocess-e2b: spawn/spawnTerminal stop validating typed spec fields
  (trust-TypeScript rule; pty-local validates its config before specs exist);
  resolveExecutable rejects separator-containing relative paths per the seam
  contract; terminal setups tracked as a Set of records.
- subprocess-e2b: PTY output push-without-backpressure is a documented
  contract (flowing consumer folds bytes; paused consumer buffers).
- fs-e2b: streamText normalizes the pinned SDK's empty-file '' return into an
  empty stream instead of throwing on getReader().
- e2b overlays: comment the one-world cwd invariant across e2b.cwd,
  workspaceRoot, and bash-local's implicit default workdir.

packages/e2b/fs-e2b/tests/filesystem.spec.ts

@@ -149,7 +149,7 @@ class FakeRemote {
         }
         return this.info(path)
       },
-      read: async (path: string, options: { format: 'bytes' | 'stream'; signal?: AbortSignal }): Promise<Uint8Array | ReadableStream<Uint8Array>> => {
+      read: async (path: string, options: { format: 'bytes' | 'stream'; signal?: AbortSignal }): Promise<Uint8Array | ReadableStream<Uint8Array> | string> => {
         this.checkAbort(options)
         if (this.nextReadError !== undefined) {
           const error = this.nextReadError
@@ -158,6 +158,8 @@ class FakeRemote {
         }
         const data = this.followed(path).node.data
         if (options.format === 'bytes') return data.slice()
+        // Pinned-SDK fidelity: a content-length-0 response returns '' even in stream format.
+        if (data.length === 0 && this.streamChunks === undefined) return ''
         const chunks = this.streamChunks ?? [data.slice()]
         return new ReadableStream<Uint8Array>({
           start: (controller) => {
@@ -375,6 +377,15 @@ describe('E2BFileSystem identity, metadata, and reads', () => {
     expect(initiallyBuffered).toBe('€')
   })
 
+  it('streams an empty file even though the pinned SDK returns a non-stream value', async () => {
+    const remote = new FakeRemote()
+    remote.file('/workspace/empty.txt', '')
+    const { fs } = await setup(remote)
+    let streamed = ''
+    for await (const chunk of await fs.streamText(await fs.resolve('empty.txt'))) streamed += chunk
+    expect(streamed).toBe('')
+  })
+
   it('cancels a remote stream when its consumer stops early', async () => {
     const remote = new FakeRemote()
     remote.file('/workspace/text.txt', 'ab')