Expand description
Workspace-level helpers for reading [segments.<id>] TOML extras.
The render-time config bag is BTreeMap<String, toml::Value>; each
segment’s from_extras constructor reads its own keys out of it.
Helpers here cover patterns that any segment can use
(warn-on-wrong-type, silent-on-absent) so segment families don’t
reinvent them.
Family-specific helpers (e.g. the rate-limit family’s
apply_common_extras and parse_*_format parsers) live next to
the family they serve, not here.
Warn-message format matches the existing convention:
segments.{id}.{key}: expected <type>; ignoring.