v0.2.1

• Add --which-file / -w flag to print the path to the detected history file
• filterLines now returns ![]const []const u8 instead of std.ArrayList for a more
  slice-oriented and allocator-flexible API
• Rename test history files to history.bash / history.bash.expected to prepare for
  multi-shell test support
• Fix typos and small code tweaks
• README: document Zig version compatibility, AUR installation, and binary downloads