seed:node spreads its ranges and inspections over worker processes

One process ran everything on one core. SEED_PROCESSES (default: the
number of cores) worker processes now each run seed:node with a share of
the slots; stop signals are passed on and crashed workers restarted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

seed-node/node.env.example

@@ -6,6 +6,8 @@ SEED_TOKEN=sn_replace_me
 SEED_RANGES=4
 # Content inspections and archive walks at once (CPU for decoding and parsing; articles come through the seed API).
 SEED_INSPECTORS=0
+# Ranges and inspections are spread over this many processes, one core each (default: the number of cores).
+# SEED_PROCESSES=4
 
 # Optional: SEED_FETCH=direct seeds with this node's own NNTP account instead of through the seed API.
 # SEED_FETCH=direct