release: v0.5.1 — sync VERSION strings with build.zig.zon

v0.5.0 shipped with VERSION constants still set to "0.1.0" in main/msg/bar,
so `zephwm --version` reported the wrong version. Bump all three to 0.5.1
and the zon version in lockstep.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zlice dev

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .zephwm,
-    .version = "0.5.0",
+    .version = "0.5.1",
     .fingerprint = 0x342fbfe9e4dd8c27,
     .minimum_zig_version = "0.16.0",
     .paths = .{ "build.zig", "build.zig.zon", "src", "zephwm-msg", "zephwm-bar", "tests", "config" },

src/main.zig

@@ -15,7 +15,7 @@ const linux = std.os.linux;
 
 extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;
 
-const VERSION = "0.1.0";
+const VERSION = "0.5.1";
 
 /// Zig 0.16: std.posix.getenv was removed. Use libc getenv via std.c.
 fn getenv(name: [:0]const u8) ?[]const u8 {

zephwm-bar/main.zig

@@ -9,7 +9,7 @@ const c = @import("c");
 
 extern "c" fn execvp(file: [*:0]const u8, argv: [*:null]const ?[*:0]const u8) c_int;
 
-const VERSION = "0.1.0";
+const VERSION = "0.5.1";
 
 // Colors (raw pixel values — no XftColor needed)
 const BG_COLOR: u32 = 0x1a1a2e;

zephwm-msg/main.zig

@@ -2,7 +2,7 @@
 const std = @import("std");
 const ipc = @import("ipc");
 
-const VERSION = "0.1.0";
+const VERSION = "0.5.1";
 
 fn usage() void {
     std.debug.print(