v0.4 (build 2, g4239919)

• Added a Command builder for arbitrary, per-invocation shell build steps
  (env.Command(inputs, output, command="...")), e.g. generating disk images or installing a bootloader.
• Added a cross-platform Copy builder.
• Added Depends as an alias for Phony, for pinning build ordering.
• Batch mapping: a builder's output= argument now accepts a callable, mapping a single-input
  builder over a list of inputs and deriving each output (generalises the %.o: %.c pattern).
• Options are now stored as variables in the generated build.ninja, and only builders actually
  used by the graph emit a rule.
• Deterministic build.ninja output: stable rule names and sorted source_glob.
• Shared/diamond dependencies are now emitted once (previously produced duplicate build statements).
• Friendlier error messages (unknown builder, missing option, missing output).
• Full type hints across the codebase, checked with mypy and ruff.
• Added an automated test suite and CI.
• Fixed for_subdir(deep_clone=True) which previously shared the parent's options.