## What changed
- The PTY Python REPL test now starts Python with a startup marker
already embedded in argv.
- The test waits for that marker in PTY output before making assertions.
## Why this fixes the flake
- The old version tried to probe the live REPL almost immediately after
spawn.
- That races PTY initialization, Python startup, and prompt buffering,
all of which vary across platforms and CI load.
- By having the child process emit a known marker as part of its own
startup path, the test gets a deterministic synchronization point that
comes from the process under test rather than from guessed timing.
## Scope
- Test-only change.
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