Add Zig 0.15 support; require Zig 0.15.0 as minimum version

[trodemaster]

Zig 0.15 introduces several breaking API changes that prevent this project from building. This PR addresses all of them.

Breaking: Zig 0.14 and earlier are no longer supported.

Build system

• build.zig.zon: set minimum_zig_version = "0.15.0"
• build.zig: addExecutable/addTest now use root_module + b.createModule() — the top-level root_source_file field was removed in 0.15
• Benchmark step disabled: zbench is not yet compatible with Zig 0.15

C import

• c.zig: pub usingnamespace @cImport(...) → pub const c_impl = @cImport(...) — re-exporting usingnamespace is no longer allowed
• All call sites updated to use @import("c.zig").c_impl; NSApplicationLoad accessed via @import("c.zig").NSApplicationLoad()

Stdlib / language

• std.ArrayList → std.array_list.Managed
• std.time.sleep → std.Thread.sleep
• stderr.reader() / .writer() → .deprecatedReader() / .deprecatedWriter()
• std.posix.empty_sigset → std.posix.sigemptyset()
• callconv(.C) → callconv(.c)

Format protocol

• Custom format methods updated to: format(self, writer: anytype) !void — comptime fmt and FormatOptions parameters removed in 0.15
• Call sites that invoke the format method use {f} specifier
• Mappings.format and Mode.format rewritten without allocation to match the error{WriteFailed}!void error set required by {f}

CI

• Both jobs updated from Zig 0.14.0 to 0.15.2

Testing

Tested on macOS 26.3.1 (arm64) with Zig 0.15.2:

• zig build test — all tests pass
• zig build -Doptimize=ReleaseFast — succeeds
• ./zig-out/bin/skhd --version — binary runs correctly
• Ran as the active skhd daemon with an existing .skhdrc — hotkeys work

[trodemaster]

Hi, thanks for reviving skhd in zig! I have been using skhd for many years. My eventual goal here with updating skhd.zig to zig 0.15.x is so I can create a macports port for skhd.zig. I have been using skhd.zig built with this branch for a week or so and it's working well for me. I did use cursor/claude to make this PR with manual testing for validation.