StatsCan: never gamble on member[0]; probe unpublished coordinates

Root cause of "selector picks a good cube, fetch dies, fallback wanders
cross-provider": dimensions with no matching branch defaulted to
members[0] (provider table order — often a suppressed cross-tab cell or
a non-aggregate member), and two of the three coordinate builders had no
retry when WDS rejected the cell.

- _select_default_member_id's terminal fallback returns a detectable
  aggregate (keyword or hierarchy-root) or None — callers go through
  _default_member_or_raise, which either discloses an arbitrary pick via
  a metadata note (paths that surface notes) or fails closed with an
  attributable statscan_required_dimension_missing supportability error.
- fetch_with_breakdown pins the requested member and routes every other
  dimension through the same selection (was hardcoded "1").
- Shared _fetch_coordinate_with_fallback probes bounded metadata-derived
  neighbours (user-explicit dimensions pinned) before failing; wired into
  fetch_categorical_data and fetch_with_dimensions with substitution
  transparency notes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hanlulong