…all views

Implements issues #3574 and #3575 from the MemCoT paper (arXiv:2604.08216).

SemanticStateAccumulator (zeph-core):
- Summarizes conceptual state after each LLM turn via a fast-tier provider
- Accumulates semantic deltas across the conversation
- Background distillation with tokio::time::timeout bound; cost-gated by
  min_assistant_chars, min_distill_interval_secs, and max_distills_per_session
- Feeds enriched query context into ReasoningMemory recall via [state] prefix
  fusion (capped at 2x query length to prevent query poisoning)
- Metrics: memcot_distill_{total,timeout_total,error_total,skipped_total}
- Span: core.memcot.distill with result=ok|timeout|error attribute
- Soft fast-tier provider validator at Config::validate startup

RecallView + recall_graph_view (zeph-memory):
- New RecallView enum: Head | ZoomIn | ZoomOut
- SemanticMemory::recall_graph_view unifies SA and non-SA graph recall paths
- ZoomIn: head edge + provenance source message (reduces chunk boundary noise)
- ZoomOut: head edge + 1-hop BFS neighbor expansion for causal context
- Three new GraphStore methods: source_message_ids_for_edges,
  source_entity_id_for_edge, bfs_edges_at_depth
- GraphFact.edge_id: Option<i64> added across all four retrieval backends

Config:
- New [memory.memcot] TOML section: enabled (default false), distill_provider,
  distill_timeout_secs, min_assistant_chars, min_distill_interval_secs,
  max_distills_per_session, recall_view

Debug dump:
- RequestDebugDump::memcot_state: Option<&str> populated when accumulator
  has non-empty state, satisfying #3574 AC#2

All features are completely invisible when memory.memcot.enabled = false:
no LLM calls, no allocations, no latency overhead. RecallView defaults to
Head view (byte-identical to prior behavior).

Note: #3575 title references ReasoningMemory::recall — this method does not
exist; the correct integration surface is SemanticMemory::recall_graph_view
as specified in APEX-MEM spec §14.1. No rename of ReasoningMemory.
AC#3 of #3575 (LoCoMo fixture) deferred to follow-up issue F1 (LoCoMo
benchmark dataset not present in repo); covered by insta snapshot tests.

Closes #3574
Closes #3575