Alpha updates can truncate persisted steering vectors when models are unloaded

[blacks1ne]

Summary

Changing steering-vector alpha while its model is unloaded can overwrite a persisted vector as a one-element vector.

Evidence

MemoryStore::user_vector_load(name, hidden_size) reads the saved blobs but ignores the stored hidden_size column and pads/truncates them to the caller-provided size.

• user_vector_set_alpha uses size 1 when no LLM is loaded, then saves the returned state.
• user_vector_set_alpha_named first loads at size 1, observes a reported hidden_size of 1, loads again at that size, then saves.

Moving an alpha slider can therefore destroy a learned LLM, 768-dimensional SD 1.5, or 2048-dimensional SDXL vector.

Proposed resolution

• Add a load-existing API that reads and validates the persisted dimension.
• Represent the caller's expected dimension as optional validation, not a replacement dimension.
• Refuse model-dimension mismatches unless an explicit migration/reset is requested.

Acceptance criteria

• Alpha/status operations with an unloaded engine preserve every vector element.
• Tests cover persisted 768-, 2048-, and arbitrary LLM-hidden-size vectors.
• A cross-model dimension mismatch returns a clear error or follows a documented migration path.