AC-1 names two inputs and says "both by fixture". The suite had the first and,
for the second, only a re-sweep of a BYTE-IDENTICAL body — which is the test
plan's other must-not-echo bullet, and cannot stand in for this one: an
identical body is quiet under both spellings of the decision, the one that
keys on the parse and the one that keys on the prose, so it cannot tell them
apart. Only an edit that changes the prose and preserves the parse can.
The new probe reorders the refs and adds sentences on either side, leaving the
set at {#90, #91}, and asserts the marker count, the thread's total echo count
and the issue-edit count all hold still. What it pins is that the marker is a
function of the PARSE and not of the prose around it — the property the whole
idempotency rests on.
Mutation-proven isolating: an echo that also re-fires when the declaration
prose moved since the last echo — quiet on identical re-sweeps, correct on
every set change — passes the pre-existing suite 304/0 and reds only here.
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