Cross-platform IPC via interprocess + Windows support fixes

- Replace Unix-socket-only server/client with interprocess local sockets:
  unchanged unix domain socket on Linux (SOCKET_API compatible),
  named pipes on Windows
- Run command lines with arguments via sh -c / cmd /C in the PTY
- Wire RunProgram args into the spawned command
- Make libc a unix-only dependency
- Skip Kitty background image on Windows (protocol unsupported)
- Drop dead pixel-size query code in kitty_gfx
- Reject left/right panel positions at config parse instead of silently
  ignoring them

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.lock

@@ -331,6 +331,12 @@ dependencies = [
  "crypto-common",
 ]
 
+[[package]]
+name = "doctest-file"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c2db04e74f0a9a93103b50e90b96024c9b2bdca8bce6a632ec71b88736d3d359"
+
 [[package]]
 name = "document-features"
 version = "0.2.12"
@@ -614,6 +620,19 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "interprocess"
+version = "2.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "069323743400cb7ab06a8fe5c1ed911d36b6919ec531661d034c89083629595b"
+dependencies = [
+ "doctest-file",
+ "libc",
+ "recvmsg",
+ "widestring",
+ "windows-sys",
+]
+
 [[package]]
 name = "itertools"
 version = "0.14.0"
@@ -1225,6 +1244,12 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "recvmsg"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3edd4d5d42c92f0a659926464d4cce56b562761267ecf0f469d85b7de384175"
+
 [[package]]
 name = "redox_syscall"
 version = "0.5.18"
@@ -1694,6 +1719,7 @@ dependencies = [
  "anyhow",
  "crossterm",
  "image",
+ "interprocess",
  "libc",
  "portable-pty",
  "ratatui",
@@ -1937,6 +1963,12 @@ dependencies = [
  "wezterm-dynamic",
 ]
 
+[[package]]
+name = "widestring"
+version = "1.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72069c3113ab32ab29e5584db3c6ec55d416895e60715417b5b883a357c3e471"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"