feat(views): grouping source helpers and the effective-grouping resolver

Phase 1 of the interactive-grouping spec: the source/resolver plumbing, no command yet. pearl--grouping-choices maps the display labels (status/project/assignee/priority/none) to Linear's issueGrouping values. pearl--grouping-display-to-value coerces and pearl--check-grouping validates, both refusing anything outside that set. Cycle is left out on purpose: it isn't in the issue drawer, so an offline regroup can't read it.

pearl--source-with-grouping records the choice under :client-group for every source type, leaving a view's own server :group intact. Choosing none removes the key, so the source falls back to the server :group or to flat. pearl--effective-grouping returns :client-group when set, else :group. That's the single read every grouping consumer routes through in phase 4, so a refresh reproduces the layout on screen.

16 tests: coercion, validation, the source helper, resolver precedence, and the :client-group header round-trip.

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