Feature: build_merge dry-run, god-node degree-drop alert, and pre-write backup

[Ns2384-star]

Version: graphifyy 0.9.5.

Problem

build_merge writes graph.json with no preview and no backup. The shrink guard (#479) protects against global shrinkage, but not against local destruction of a hub: the graph can grow overall while a god node drops to 0 edges (lived: a 174-edge hub wiped by replace-on-re-extract after an entity-ID mismatch — total node count went up that run).

Proposal

1. build_merge(dry_run=True) → report of what would be pruned/replaced (nodes, edges, per-source counts) without writing.
2. Post-merge automatic alert when any node with ≥N edges loses >50% of its degree.
3. Automatic backup of graph.json before write (a single rolling copy is enough).

We've implemented (2) and (3) as a wrapper script around every update; they feel like they belong in the tool. Related: #834 (dated-folder backup, closed).