Fix stdio frame interleaving under backpressure

[rohitsalla]

Fixes #263

Summary

• Serialize StdioTransport.send() calls in FIFO order.
• Keep the existing nonblocking partial-write and EAGAIN retry behavior.
• Add a regression test that forces pipe backpressure and verifies concurrent sends preserve newline-delimited frame boundaries.

Root cause

StdioTransport is an actor, but its write loop suspends while retrying EAGAIN. Actor reentrancy allowed a second send() call to write while the first frame was only partially written, splicing one JSON-RPC frame into another.

Each send now waits for the previously enqueued send to complete before entering the write loop. A failed send does not prevent later queued sends from running.

Verification

• Real nonblocking-pipe reproduction before the change: ordered=false, with the second frame's newline inside the first payload.
• The same reproduction after the change: ordered=true, first newline at the expected 524288-byte boundary.
• The production module compiled successfully through the reproduction executable.
• git diff --check passes.

The full swift test target could not run locally because the installed macOS Command Line Tools lack the repository's existing Testing module and have a Swift compiler/SDK patch mismatch. The pull request CI matrix provides the required Swift 6.1 macOS and Linux coverage.