Fix fixture declarations not propagating to auto-included shared example groups (#33)

When shared example groups are auto-included via `config.include_context`
with metadata filters, RSpec creates those child groups during
configuration — before the spec file's `fixture` call sets the
declaration on the parent group's metadata. Since RSpec snapshots parent
metadata into child groups at creation time, the child's metadata never
includes the fixture declaration, and the `prepend_before` hook never
fires for those examples.

The fix uses RSpec's own `update_inherited_metadata` to propagate the
fixture declaration to any child groups that already exist when `fixture`
is called. This is the same mechanism RSpec uses internally to push
metadata updates to descendants.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>