0.3.0 — the broken class, directory walking

Added

• The broken class. scan now reports U+FFFD and the private use areas
  alongside look-alikes, compat characters and invisible ones — damage that cannot
  be repaired, only reported. A private use code point earns its place in the
  report precisely because nothing defines what it looks like: the font doing the
  rendering decides, and it can come out as a box, as nothing at all, or as a
  glyph that means something else. (b17902d)
• scan walks directories. A corpus is a tree, not an argument list.
  Dot-entries stay out, so .git does not end up in the report; a non-text file
  found by walking is skipped and counted, while a non-text file you named is
  still an error — pointing at a PDF is a mistake, walking past one is not.
  (b17902d)
• The scan footer prints once per run instead of once per file, which starts to
  matter the moment you point it at a directory. (b17902d)

Tests

• The four compat characters the roadmap named are pinned with measured values:
  full-width Ａ → A, ideographic space → space, U+F900 → 豈, NBSP → space.
  (b17902d)

No change to fold(), to check, or to the equivalence table. Exit codes and
check output are unchanged.