core/process: cross-platform PTY via backend and pywinpty

POSIX openpty/fork and Windows ConPTY; win32 marker dep plus typings stub.

typings/winpty.pyi

@@ -0,0 +1,30 @@
+"""Stubs for pywinpty (ConPTY) matching winpty.PtyProcess runtime API."""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+from typing import Any
+
+class PtyProcess:
+    exitstatus: int | None
+
+    @classmethod
+    def spawn(
+        cls,
+        argv: str | Sequence[str],
+        cwd: str | None = None,
+        env: dict[str, str] | None = None,
+        dimensions: tuple[int, int] = (24, 80),
+        backend: int | None = None,
+    ) -> PtyProcess: ...
+    def isalive(self) -> bool: ...
+    def read(self, size: int = 1024) -> str: ...
+    def write(self, s: str) -> int: ...
+    def terminate(self, force: bool = False) -> bool: ...
+    def kill(self, sig: int | None = None) -> None: ...
+    def close(self, force: bool = False) -> None: ...
+    def wait(self) -> int | None: ...
+    def fileno(self) -> int: ...
+    def isatty(self) -> bool: ...
+    def setwinsize(self, rows: int, cols: int) -> None: ...
+    def getwinsize(self) -> tuple[int, int]: ...