feat(falkordb): implement native edges for relations

[dudizimber]

resolves #41

Summary

Remodels FalkorDBGraphStore so relations are stored as native FalkorDB edges
between object nodes instead of standalone AGRelation nodes with source/target
id properties. The projection becomes a real, traversable graph — connected in the
FalkorDB Browser and usable with native variable-length paths and graph algorithms —
without losing dangling-relation support or cascade semantics.

This is a behavior-preserving internal remodeling of FalkorDBGraphStore only. The
GraphStore contract, the in-memory store, and the event log are untouched, and
because the store is a disposable projection of the event log there is no data
migration — existing runs re-replay into the new layout.

What changed

• Relations are native edges with a single fixed relationship type.
  (s)-[:AGRelation {id, type, data, provenance}]->(t). The relation's type
  (plus id/data/provenance) rides on the edge as properties; source/target
  are no longer stored — they fall out of the endpoints, making the graph the single
  source of truth for connectivity. Traversal by kind is a property filter:
  MATCH (s)-[r:AGRelation {type:'knows'}]->(t).
• Shared :AGNode endpoint label. Every endpoint carries a structural :AGNode
  label; real objects additionally carry :AGObject. put_relation MERGEs endpoints
  on :AGNode (index-backed, no full scans) and put_object promotes the same node
  in place with SET o:AGObject.
• Dangling relations via derived placeholders. A dangling endpoint is exactly
  :AGNode AND NOT :AGObject — placeholder-ness is derived, never a stored flag.
  When the object later arrives the node is promoted in place; when a relation is
  removed, any endpoint left as an orphaned placeholder (no remaining edges) is
  garbage-collected.
• Cascade-on-removal unchanged. The core.graph projector still deletes a removed
  object's relations (now edges), so behavior is identical across every GraphStore.
• Indexes. AGNode(id) (backs endpoint MERGE), AGObject(id) (object reads),
  relationship-property indexes AGRelation(id) and AGRelation(type), and
  AGPatch(id). A :AGNode:AGObject node appears in both label indexes.

Security

Because the relationship type is a fixed literal (AGRelation), every value stays a
bound $param — no string interpolation and no injection surface, even for
user-supplied relation type strings.

Tradeoff

All edges share the AGRelation label and the relation kind lives in the type
property, so Browser styling/filtering keys off the property rather than the edge
label. This is the price of keeping the write path injection-free and matches the
type-as-attribute shape the projection already uses.

Testing

• Added FalkorDB tests covering placeholder promotion (relation written before its
  object), placeholder cleanup on relation removal, and native-edge round-tripping;
  the existing GraphStoreConformance suite passes unchanged (same observable
  behavior).
• Live FalkorDB run (Docker container) green; persistence/fork/graph suites pass.

Docs / changelog

• Updated docs/guides/using-falkordb.md "How entities are stored" section for the
  native-edge model, placeholders, and Cypher inspection snippets.
• Added an [Unreleased] entry to CHANGELOG.md.