Concurrent index: reverse-edge lists grow under update churn until consolidation #469

Summary

With SVS_SYNC_KIND_FINE_GRAIN, the concurrent index's reverse-edge index
(ReverseEdges, include/svs/concurrent/reverse_edges.h) only ever gains
entries during inserts and updates. Stale entries are pruned only by
consolidation. A caller that reuses deleted slots with
svs_index_dynamic_update_points + svs_index_dynamic_replace_id (#467, #468)
rarely needs to consolidate, so these lists keep growing. The growth shows in
svs_index_get_memory_usage and in graph_bytes of
svs_index_get_memory_breakdown, because the C API adds reverse_edges_bytes
to graph_bytes (bindings/c/src/svs_c.cpp:678).

Where it comes from

The code below is on main (added in #369) and unchanged by #467 or #468;
line numbers are at 9330e49, the head of #468.

• replace_node_locked (include/svs/concurrent/graph.h, around line 503)
  records every new out-edge and, for every dropped out-edge, records the
  reverse pairing. It never removes an entry. The exact variant is commented
  out ("[Exact list] ... reverse_edges list would be exact for single
  thread. Need investigation for concurent case.").
• ReverseEdges stores each list as a std::vector, so capacity is never
  returned even when consolidation removes entries.

Measurement

Single-threaded, through the C API. L2, FLOAT32, dimension 8,
graph_max_degree 64, construction window 128, 500 live vectors. Each round
deletes 100 vectors, then inserts 100 new ones into the deleted slots
(update_points on the deleted id, then replace_id to the new id). No
consolidation runs.

data_bytes stays flat, so every slot is reused. All of the growth is in
graph_bytes, about 130 extra entries per node, roughly twice
graph_max_degree. After that it keeps rising slowly, by about 1.6 KB per
5,000 inserts.

Request

Could the reverse-edge index stay bounded without a full consolidation?
For example:

• remove the reverse entry when an edge is dropped (the commented-out exact
  variant), if it can be made safe for concurrent writers; or
• prune and shrink_to_fit a node's list when update_entries rewrites that
  node, since it already holds the node's lock and repairs its edges.

Without one of these, an index whose inserts reuse deleted slots grows to
about twice its graph size under steady churn, even though it never adds a
slot.