Realize relational-signature identity at write time (design gap #1)

[StigNorland]

DESIGN.md's central claim: concept identity = relational signature (language-agnostic, structural). But node matching at write time is pure surface form (char-bigram Jaccard on labels). Only merging uses the relational signature. The language-agnostic promise is not realized where it matters most.

• Prototype signature-aware retrieval/matching in write()
• Evaluate effect on LoCoMo vs. surface-form baseline

Refs: src/library/graph.py:527 (_label_similarity), :507 (_signature_similarity). See MEMORY.md gap #1.

[StigNorland]

Reframed by the v2 graph design (#18, paper/graph-model-v2.md): identity at write time is matching (entities/facts are singular, never merge); the aha-moment merge cascade is concept-only. The 'signature identity' goal now lives inside the v2 bottom-up concept-formation sub-workstream. Tracking under #18.