runtime: engram_edges_json — read edges without a whole-graph file round trip

/api/graph/edges answered a read query by calling engram_save() to serialize
the ENTIRE graph to disk (128 MB) and then fs_read-ing it back. Two defects in
one line, and both bit production on 2026-08-15:

  1. The path it wrote was ~/.neuron/engram/snapshot.json — the engram
     server's CANONICAL store. A READ route overwriting the persistence
     owner's canonical file. This defect had been fixed once (export moved to
     a scratch path); it came back when the hand-written dispatch block was
     replaced by @route dispatch and the unfixed copy is the one that
     survived the merge.
  2. Cost: a full snapshot write, a 128 MB read, and a parse of the whole
     graph, per request, to return a bounded slice.

Calling it tonight overwrote the canonical snapshot and immediately preceded
an engram crash loop.

engram_edges_json(limit, offset) is the builtin that route's own TODO asked
for ("Future: add an engram_edges_json() builtin and drop the file round trip
entirely"). It walks g->edges directly and emits every persisted field.

limit <= 0 defaults to 1000, not unbounded: this is the endpoint that fell
over, and an unbounded default would preserve the failure mode under a new
name. Callers page explicitly.

Registered in codegen.el's builtin_arity (both plain and __ spellings) and
wrapped in el_seed.c per the project's C-builtin recipe.

lang/el-compiler/src/codegen.el

@@ -2764,6 +2764,7 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "__engram_node_full_in") { return 9 }
     if str_eq(name, "__engram_connect_in") { return 5 }
     if str_eq(name, "__engram_scan_nodes_json") { return 2 }
+    if str_eq(name, "__engram_edges_json") { return 2 }
     if str_eq(name, "__generate") { return 1 }
     // Filesystem
     if str_eq(name, "fs_read") { return 1 }
@@ -2862,6 +2863,7 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "engram_get_node_by_label") { return 1 }
     if str_eq(name, "engram_search_json") { return 2 }
     if str_eq(name, "engram_scan_nodes_json") { return 2 }
+    if str_eq(name, "engram_edges_json") { return 2 }
     if str_eq(name, "engram_neighbors_json") { return 3 }
     if str_eq(name, "engram_activate_json") { return 2 }
     if str_eq(name, "engram_stats_json") { return 0 }