refactor(pg-delta): move execution phase classification onto BaseChange.phase

[Copilot]

This refactor removes phase classification from sort-layer helpers and colocates it with change objects via BaseChange.phase, while preserving existing execution behavior. It also renames the phase value from create_alter_object to forward across pg-delta.

• Phase ownership moved to change model

  • Added Phase = "drop" | "forward" and default phase getter to BaseChange.
  • Made phase selection total over operation (create/alter/drop) with no fallback default.

• Layering cleanup

  • Deleted core/sort/utils.ts (getExecutionPhase, old Phase, table-specific instanceof logic).
  • Moved isMetadataStableId to core/stable-id.ts so BaseChange and sort code can share it without sort→object coupling.

• Load-bearing table alter overrides

  • Added phase overrides in:
    • AlterTableAlterColumnDropDefault → drop
    • AlterTableAlterColumnDropIdentity → drop
    • AlterTableAlterColumnType → forward for custom type, otherwise drop

• Sort pipeline updated to consume change.phase

  • Updated logical-sort.ts and sort-changes.ts to partition on change.phase.
  • Replaced create_alter_object buckets/maps with forward.

• Top-level drop predicate left intact (with clarification)

  • Kept catalog.diff.ts object-drop filtering logic (operation === "drop" && scope === "object") and added a clarifying comment that this is intentionally different from phase checks.

• Docs and tests

  • Added CONTEXT.md at repo root with canonical terms: catalog, change, phase, plan.
  • Added base.change.test.ts matrix coverage for default phase behavior, metadata-only drops, and mixed metadata+object drops.
  • Added focused phase tests in table.alter.test.ts for each override case.

// BaseChange default phase classification
get phase(): Phase {
  switch (this.operation) {
    case "drop":
      return "drop";
    case "create":
      return "forward";
    case "alter": {
      const dropsRealObject = this.drops.some((id) => !isMetadataStableId(id));
      return dropsRealObject ? "drop" : "forward";
    }
  }
}

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