v1.7.0

Add nested subcommand support (sub-usage_t per command)

A usage_t may now declare `commands`: a declared-order vector of
{name, shared_ptr<usage_t>}. When the first positional argument matches a
command name, the rest of the command line is handed to that command's own
usage_t and parsed against ITS options (recursively - a sub-usage can have its
own commands). Global options before the command still parse against the parent.
A null entry is a raw passthrough: the command is recognised but its arguments
are left unparsed, retrievable via tail() (for wrapping foreign flag sets like
`docker run`).

shared_ptr keeps usage_t an aggregate, so designated-init (`.args = {...}`) still
works and the sub-usages are released with the parent - no destructor (which
would break the aggregate). args_t gains a default ctor for command usage_t's
whose args are filled on dispatch.

New API: commands member, subcommand()/sub()/tail(). Empty commands == the
previous single-level behaviour, bit for bit (verified). Example + README updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ax7adsbWbibrSvpAGr7DTJ