fix: segfault on Python 3.14 in libei variadic function call

[davidselassie]

Summary

Set argtypes on ei_seat_bind_capabilities — a variadic C function
whose argtypes were intentionally left unset because ctypes historically
didn't require them. On Python 3.14 this causes a segfault (exit code
139) during session start, as ctypes misinterprets the variadic arguments
at the C ABI level without the fixed parameter declaration.

Motivation

uvx install kwin-mcp does not respect .python-version — it uses
whatever Python satisfies requires-python (>=3.12). Users on systems
with Python 3.14 as default (e.g. Fedora 43) hit this crash immediately
on session start. The fix sets argtypes to just the fixed parameter
(the seat pointer); the variadic arguments at the call site are already
wrapped as ctypes instances and pass through correctly.

How to Test

1. uv python pin 3.14 && uv sync
2. Revert the fix in src/kwin_mcp/input.py
3. echo "session_start" | uv run python -m kwin_mcp.cli → exit code 139 (SIGSEGV)
4. Restore the fix, re-run → session starts normally
5. uv python pin 3.12 && uv sync to restore

Checklist

• uv run ruff check . passes
• uv run ruff format --check . passes
• uv run ty check passes
• CHANGELOG.md updated (if user-facing change)
• README.md updated (if new tools or changed behavior)