Feature: Unified context fragment model hierarchies
  The core ContextFragment, FragmentProvenance, ContextBudget, and
  ContextPayload types extend their CRP base counterparts, ensuring
  isinstance compatibility across the model hierarchy.

  Background:
    Given the unified model modules are available

  # ---------------------------------------------------------------
  # isinstance compatibility
  # ---------------------------------------------------------------

  Scenario: Core FragmentProvenance is instance of CRP FragmentProvenance
    Given a core FragmentProvenance with resource_uri "project://test"
    Then the provenance should be an instance of CRP FragmentProvenance
    And the provenance resource_type should be "unknown"
    And the provenance strategy should be empty

  Scenario: Core FragmentProvenance inherits CRP strategy field
    Given a core FragmentProvenance for strategy "tier_retrieval" with resource_uri "project://test"
    Then the provenance strategy should be "tier_retrieval"
    And the provenance resource_type should be "unknown"

  Scenario: Core ContextFragment is instance of CRP ContextFragment
    Given a core ContextFragment with uko_node "project://app/main.py" and content "hello"
    Then the fragment should be an instance of CRP ContextFragment
    And the unified fragment tier should be "warm"
    And the unified fragment fragment_id should be non-empty

  Scenario: Core ContextFragment has ULID fragment_id by default
    Given a core ContextFragment with uko_node "project://test" and content "test content"
    Then the unified fragment fragment_id should be 26 characters

  Scenario: Core ContextFragment created_at is auto-populated
    Given a core ContextFragment with uko_node "project://test" and content "test content"
    Then the unified fragment created_at should be a recent datetime

  # ---------------------------------------------------------------
  # Immutability (frozen)
  # ---------------------------------------------------------------

  Scenario: Core ContextFragment is frozen
    Given a core ContextFragment with uko_node "project://frozen" and content "frozen"
    When I attempt to mutate the fragment uko_node
    Then a frozen mutation error should be raised

  Scenario: Core ContextBudget is frozen
    Given a core ContextBudget with max_tokens 4096 and reserved_tokens 512
    When I attempt to mutate the budget max_tokens
    Then a frozen mutation error should be raised

  Scenario: CRP FragmentProvenance is frozen
    Given a CRP FragmentProvenance with resource_uri "test://crp"
    When I attempt to mutate the CRP provenance resource_uri
    Then a frozen mutation error should be raised

  # ---------------------------------------------------------------
  # Equality across hierarchy
  # ---------------------------------------------------------------

  Scenario: Two core fragments with same data are equal
    Given two core ContextFragments with identical fields
    Then the two fragments should be equal

  Scenario: Core ContextBudget strict validation rejects equal reserved and max
    When I create a core ContextBudget with reserved_tokens equal to max_tokens
    Then a validation error should be raised for strict budget