graph.json (the clustered `to_json` write and the `--no-cluster`/merge raw
dumps) and manifest.json were written with a direct `open()`/`write_text`, so a
crash, kill, or disk-full mid-write left a truncated, unparseable file that the
next load or `detect_incremental` then failed on.
Add `write_text_atomic`/`write_json_atomic` in graphify.paths (temp file in the
same directory + `os.replace`; JSON is streamed into the temp, not materialized
as one string) and route the graph.json writers (export.to_json,
cli._prune_graph_json_sources, the merge driver) plus detect.save_manifest
through them. The helper preserves the destination's mode (an atomic replace
never tightens 0644 to mkstemp's 0600), writes through a symlinked destination
(shared-output setups), and falls back to copy-then-delete on a Windows
os.replace lock — matching graphify.cache's existing atomic writer. On failure
the previous file is left intact and the temp removed. Not a power-loss
durability guarantee (no fsync, consistent with the rest of the codebase).
