Trace store — completing the tap-subscriber cut
Completes the tap-subscriber cut (#283) around its trace store, in the wake of that cycle's fieldtest (findings filed as #309/#311; #307/#308 filed at the cycle's close). #309: a plain aura run records declared taps but neither echoes nor documents the trace handle, so the recorded trace is discoverable only by guessing the default trace-dir name. #311: two different blueprints defaulting to the same blueprint name write into the same trace dir, orphaning still-chartable tap files from the earlier run; the issue records three candidate contracts (clear the dir, reject the collision, or ratify-and-document the hazard). #308 migrates the legacy Recorder surface onto the TapTraceWriter delivery seam (landed with #283) so a single delivery shape remains. #307 replaces the JSON on-disk trace encoding with a compact binary format plugging into that same seam — JSON does not scale to multi-million-point series. Order: the two small honesty fixes first (#309, #311), then the structural levers (#308, #307).
Done: a run's traces are discoverable from its output, the same-name collision has a decided contract, and one delivery shape with a scalable encoding remains.
