Move only: function bodies are unchanged, cut from cshell.c by line
range. Each module gets a header with just what other modules use;
everything else stays static. Shell-wide state is defined in main.c
and declared extern in shell.h.

The Makefile builds each file separately, with -MMD -MP for header
dependencies. The binary grows from 25040 to 25160 bytes (less
inlining across files). Behavior checked against the old binary:
make check, 25 -c cases, and an interactive pty session.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>