Named echo effect

echo-effect.scp requests the named Echo operation, then receives either (1, value) or (0, problem) through the named echo receiver. Both branches return the retained environment alongside the received value. The source does not execute IO or look up a handler itself.

The existing focused runner exercises successful/failed handlers and missing operation/receiver names through the lexical compiler and shared effect driver:

result/bin/tricu eval test/consumer_echo_lexical.tri --store "$(mktemp -d)" \
  --io --allow-read demos/consumers/echo --format tree

It prints echo-lexical-ok, followed by the IO return value t. The interpreter and earlier prepared-worker comparison lives in test/consumer_base_effect_protocol.tri. See the language contract for the general worker/effect boundary; these are source-running checks, not deployed startup measurements.