refactor(api): eliminate duplicate traps, harden error handling & telemetry

Phase 1 - Structural:
- Remove api_exit_script() and 5 inline traps from build.func
- error_handler.func is now the sole trap owner via catch_errors()
- Update api.func comment reference (api_exit_script -> on_exit)

Phase 2 - Quality:
- Add stop_spinner() + cursor restore to error_handler(), on_interrupt(),
  on_terminate(), on_hangup() to prevent spinner/cursor artifacts
- Enhance _send_abort_telemetry() with error text (last 20 log lines),
  duration calculation, and 2 retry attempts (was fire-and-forget)
- Harden json_escape() to also strip DEL (0x7F) character

misc/api.func

@@ -312,7 +312,8 @@ json_escape() {
   s=${s//$'\r'/}
   s=${s//$'\t'/\\t}
   # Remove any remaining control characters (0x00-0x1F except those already handled)
-  s=$(printf '%s' "$s" | tr -d '\000-\010\013\014\016-\037')
+  # Also remove DEL (0x7F) and invalid high bytes that break JSON parsers
+  s=$(printf '%s' "$s" | tr -d '\000-\010\013\014\016-\037\177')
   printf '%s' "$s"
 }
 
@@ -982,7 +983,7 @@ EOF
   fi
 
   # All 3 attempts failed — do NOT set POST_UPDATE_DONE=true.
-  # This allows the EXIT trap (api_exit_script) to retry with 3 fresh attempts.
+  # This allows the EXIT trap (on_exit in error_handler.func) to retry.
   # No infinite loop risk: EXIT trap fires exactly once.
 }