Support non-git diffs from stdin or two files

[alchemmist]

Closes #140.

Makes lumen diff render diffs without a repository:

arc diff | lumen diff --stdin      # or: lumen diff -
lumen diff --files old.txt new.txt

The unified diff is parsed into the existing FileDiff pipeline, so highlighting, sidebar, search and annotations work as usual.

Notes:

• --stdin shows the hunks and their context (a patch), not the whole file.
• Three-way (diff3) mode is not included.
• When a diff is piped in, stdin is reconnected to the terminal so the viewer can still read input (via stdout/stderr, since macOS /dev/tty cannot be registered by crossterm's kqueue reader).

[alchemmist]

@jnsahaj hello! Can you check it please