discover_start's stdout is its result channel (read via command
substitution in process_symbol), but the probes inside it stream the
cBot's "[BOT] …" progress on stdout too. That noise leaked into the
captured value, so start_idx parsed to 0 and the resume walk began at
year 0 -> an unparseable "--start=01/01/0" backtest that retried out and
hard-aborted the whole run. Only a brand-new symbol (no files on disk yet)
hits this path, so it stayed invisible until AUDUSD was added.
Redirect every probe call in discover_start to stderr (1>&2) so its
stdout carries only the "YEAR MONTH" result, and document the contract.
Add scripts/test-export-runner.sh: source-level tests that stub probe_run
to emit the same stdout noise and assert a clean result (RED without the
redirect). Make SYMBOL_FILE overridable for scoped runs/tests.
