Narrow SemanticMemory generic from LlmProvider to EmbedFn

[bug-ops]

Part of #391

Problem

SemanticMemory<P: LlmProvider> only uses P::embed(), but carries the full LlmProvider generic through Agent → MemoryState. This causes monomorphization overhead.

File: crates/zeph-memory/src/semantic.rs line 58

Proposed Solution

Accept Arc<dyn EmbedProvider> (narrower trait) or EmbedFn closure instead of full LlmProvider.

Priority: P3 | Effort: Large