pxx 2.5.1

Fixed

• Embedding-space versioning for observation memory (fail-closed on embedder swap).
  Stored vectors carried no record of which embedder produced them, so changing the memory
  embedder (or the roles.embed model) silently made search() compute cosine across
  incomparable spaces — confident garbage with no error, and worst in observation memory,
  which accumulates for months and is never reindexed. The store now stamps its
  embedding-space identity in a meta table, and set_embedder fails closed with
  EmbeddingSpaceError when a different identity is attached over existing vectors —
  targeting the dangerous same-dimension / different-model case (a plain dimension
  mismatch already threw). Recovery: MemoryStore.reset_embedding_space() clears the
  vectors + stamp and detaches the embedder (re-add observations to re-embed under the new
  embedder — it does not auto-reindex), or repoint to the original embedder. Empty stores
  adopt a new embedder freely; pre-versioning
  stores with vectors assume the current embedder (logged warning, not a hard fail).
  Embedders now expose an identity (HashEmbedder → hash:dim=N, OllamaEmbedder →
  ollama:<model>). This is the retrieval analogue of the content-truthfulness gate —
  refusing to answer from a corrupt index is refusing to fabricate, not judging. +10 tests
  including a same-dim/different-model negative control. Prerequisite for the forthcoming
  repo code index; flagged by PSAIOS in coordination.