Concrete [Shell] implementations for mk-core.

Provides ShShell (POSIX /bin/sh -ec), CustomShell (user-configurable via $MKSHELL), and DuckShell (embedded duckscript, feature-gated behind the duckscript feature). All implement the mk_rs_core::shell::Shell trait.

Design

ShShell wraps std::process::Command with /bin/sh -ec, passing environment variables via cmd.env(). Recipe text is supplied as the -c argument.

Quote escaping. sh single-quotes with '...' (embedded ' escaped as '\''). rc was originally planned but is not shipped — sh + $MKSHELL covers all practical use cases, and rc would require an rc binary on the host.

find_unescaped scans for a character outside of shell quotes. Used by the attribute parser in mk-core to detect quoted = characters in assignment values. The escaping rules are shell-specific (backslash in sh, double-quote in rc).