Widen instance key to (definition_id, child_id, due_date, window) (D-008)

Per IMPLEMENTATION-ORDER.md 1.2: quest_instances gains a NOT NULL
window column (CHECK-constrained to the const window spellings) and
the table-level UNIQUE constraint widens to the full fan-out tuple.

- schema.py: instance table body extracted to QUEST_INSTANCES_TABLE_SQL
  shared with the rebuild migration; uniqueness docstring rewritten for
  the widened key.
- migrations.py: migration 5 rebuilds a pre-window quest_instances via
  the canonical procedure; migrated rows are pinned to the documented
  default 'morning' window (pre-window rows were whole-day obligations;
  future dates are regenerated with real windows by materialization),
  preserving ids so completion_events keep pointing at the same
  instances; PRAGMA foreign_key_check verified inside the transaction.
- dao_instances.py: upsert takes required keyword window and is
  idempotent on the widened tuple; the immutable-once-completed check
  and the read-back get() are keyed on the full tuple; window spelling
  validated via dao_rules._validate_window.
- tests: schema shape (window column, widened autoindex columns),
  twice-daily and shared-assignee fan-out cases at both the DDL and DAO
  layers (incl. idempotent re-runs), cross-child same-key allowance,
  window CHECK/NOT NULL constraints, v2 seed frozen to the true
  version-2 shapes and its migration assertions widened.

tests/test_dao_rules.py

@@ -927,7 +927,7 @@ def test_definition_assignment_leaves_history_rows_alone(
         # this table, but its row shape proves assignment isolation).
         await database.execute(
             "INSERT INTO quest_instances (definition_id, child_id, "
-            "due_date, generated_at) VALUES (?, ?, ?, ?)",
+            "window, due_date, generated_at) VALUES (?, ?, 'morning', ?, ?)",
             (definition.id, child.id, "2026-09-15", NOW),
         )
         await definitions.add_assignee(definition.id, other.id)