fix(input): use CFUNCTYPE to avoid variadic ABI SIGSEGV in ei_seat_bind_capabilities

[AgainPsychoX]

Problem

session_connect crashes with SIGSEGV inside ei_seat_bind_capabilities on Python 3.14 + libei 1.5.0 (reported in #29 for Fedora 43 / Bazzite).

Root cause: the original code called ei_seat_bind_capabilities as a raw ctypes variadic function. Raw variadic calls via ctypes/libffi are ABI-fragile — on Python 3.14 the register/stack placement is wrong, causing a crash inside the C function.

A secondary bug: the sentinel was c_void_p(None) (NULL pointer), but libei's loop terminator is while ((cap = va_arg(args, enum ei_device_capability)) > 0) — it checks cap > 0, not a pointer-NULL. The correct sentinel is c_int(0).

Fix

Build a fully-typed ctypes.CFUNCTYPE for the exact argument list (seat pointer + N capability uints + int sentinel). This gives libffi explicit register/stack placement for every argument, eliminating the ABI ambiguity regardless of Python or libffi version.

FuncType = ctypes.CFUNCTYPE(
    None,
    ctypes.c_void_p,                     # seat
    *([ctypes.c_uint] * len(bind_list)), # capabilities
    ctypes.c_int,                        # sentinel 0
)
FuncType(_libei.ei_seat_bind_capabilities)(seat, *bind_list, 0)

Testing

Tested on Manjaro (libei 1.4.x) via both the CLI (uv run python -m kwin_mcp.cli) and live MCP session — no regression, session_connect succeeds and input injection works correctly.

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🤖 This issue was analysed, fixed, and verified end-to-end using Claude Code (claude-sonnet-4-6) running kwin-mcp as a live MCP server — the fix was tested by actually connecting a session through the patched code.

Fixes #29