Zig 0.16.0 compatibility - std.fs.cwd() and related APIs removed

[slavakurilyak]

Gremlin fails to build with Zig 0.16.0-dev due to breaking changes in the standard library's filesystem APIs.

Affected APIs:

• std.fs.cwd() → std.Io.Dir.cwd() (requires Io instance)
• std.fs.openFileAbsolute() → std.Io.Dir.openFileAbsolute(io, ...)
• std.fs.realpath() → std.Io.Dir.realPath(io, ...)
• std.fs.realpathAlloc() → std.Io.Dir.realPathFileAlloc(dir, io, ...)

Affected Files:

• src/parser/entries/buffer.zig
• src/parser/parser.zig
• src/parser/fs/paths.zig
• step.zig

Migration Required:
All file operations now require an Io instance parameter. This requires threading an Io through function signatures or using b.graph.io in build contexts.

Zig Version:

• Currently supports: 0.15.x
• Fails on: 0.16.0-dev.2596+469bf6af0

See: https://ziglang.org/download/ (Zig 0.16.0 introduced event-driven I/O with std.Io)