Implement accessor methods on Holon for descriptor and predecessor

[evomimic]

This enhancement replaces the descriptor and predecessor fields on Holon with accessor functions (get/with).

For testing reasons, it is recommended this issue be worked in a subbranch of Issue #118.

Dependencies

• [x] Issue #95

Current State

Both of the fields above are, at best, caches of relationships whose source of truth is in the holon's _relationshipmap. The DESCRIBED_BY relationship relates a holon with its type descriptor and the PREDECESSOR relationship relates a holon to its predecessor.

Proposal

1. Drop the descriptor and predecessor fields from Holon.
2. Add get_descriptor and get_predecessor methods to HolonReference
3. Add with_descriptor and with_predecessor methods to StagedReference

In holon.rs

• [x] Drop the descriptor and predecessor fields from the definition of Holon

In holon_reference.rs

• [x] Add get_descriptor(&self)->Result<Option<HolonReference>, HolonError> method.
• [x] Add get_predecessor(&self)->Result<Option<HolonReference>, HolonError> method.

Implementation Notes:

Both of the above should:

• delegate the call to get_related_holons method on their variant, passing CoreSchemaRelationshipTypeName::DescribedBy.to_string() for DESCRIBEDBY relationship name and CoreSchemaRelationshipTypeName::Predecessor.to_string() for PREDECESSOR relationship_ name
• convert the resulting collection to a single HolonReference (or None) and
• return an Error if more than one descriptor or predecessor are found.
• Otherwise, return the HolonReference

In staged_reference.rs

• [x] Add with_descriptor(&self, context: &HolonsContext, descriptor_reference: HolonReference)->Result<Self,HolonError> method for Holon.
• [x] Add with_predecessor(&self, context: &HolonsContext, predecessor_reference: HolonReference)->Result<Self,HolonError> method for Holon.

Implementation Notes:

• Both methods should first ensure the Holon _isaccessible for Write.
• Currently, Holon's are only allowed to have a single descriptor and (at most) a single predecessor. So these methods will need to first remove any holon related through the relationship in question.
• Both methods should leverage the implementation of _add_relatedholons and remove_related_holons, but these methods must first be implemented via Issue #95.

Testing

These methods will NOT be exposed as separate dances as they are really just guest-side convenience functions. Thus, they cannot be tested directly via dance tests. However, they can be tested indirectly as part of Issue #118.

Definition of Done

• [ ] This issue may be closed when testing for Issue #118 demonstrates these methods behave correctly.

[evomimic]

@dauphin3 hey zeek... the above design references the CoreSchemaRelationshipTypeName::Predecessor.to_string() name which I later realized I hadn't yet defined. I added it to PR #146 which I just pulled into main. You can grab crates/coordinator/core_schema/src/relationship_type_loader.rs from main to give you access to that name in your branch.

[evomimic]

@dauphin3 : I intend to resolve the circular dependency problem in a separate issue. Please just use the hard-code relationship names in #143 to bypass the issue and unblock your progress towards completing this issue.