fix: use ASCII tmux format separator on Windows

[arnoliudaxia]

Problem

With native Windows tmux 3.6a-win32 and libtmux 0.62.0, list_sessions fails with ValueError: zip() argument 2 is shorter than argument 1. The default U+241E format separator is replaced by literal question marks in tmux stdout. libtmux expects 136 fields but splitting on the original separator yields only one.

Fix

• Set an ASCII LIBTMUX_TMUX_FORMAT_SEPARATOR default during package initialization on Windows, before libtmux imports and caches it.
• Preserve explicit environment overrides and leave other platforms unchanged.
• Document the compatibility default and add isolated-process regression tests for platform selection, overrides, import timing, and format parsing after lossy encoding.

Validation

• With the external separator override removed, the patched package successfully listed existing sessions on real native Windows tmux 3.6a-win32 with libtmux 0.62.0.