Composite gains doc: Option<String> (the prose twin of name, a C23 debug
symbol): with_doc/doc() on Composite, a doc knob on GraphBuilder, a
Tier-1 additive-optional CompositeData field (no format-version bump;
absent-field documents keep their exact bytes), identity-stripped like
the name. The graph model emits an optional trailing doc fragment in
both scopes; json_str is hardened for free text (\n/\t/\r named,
control chars as \u00XX — the doc is the first multi-line value through
it). e2e: a hand-authored doc'd blueprint renders with the doc embedded;
the doc moves the content id but never the identity id.
refs #125
