export-runner: fix new-symbol discovery aborting with a year-0000 backtest

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.

scripts/export-runner.sh

@@ -31,7 +31,7 @@ BOT_FILE="MS_SimpleExport.algo"
 PWD_FILE="/mnt/Robots/password.txt"          # container-internal path (console reads it)
 IMAGE="ghcr.io/spotware/ctrader-console:latest"
 HOST_ROBOTS_PATH="$REPO_ROOT/runtime/Robots" # holds MS_SimpleExport.algo + password.txt
-SYMBOL_FILE="$REPO_ROOT/runtime/ScrapeSymbols.txt"
+SYMBOL_FILE="${SYMBOL_FILE:-$REPO_ROOT/runtime/ScrapeSymbols.txt}"  # overridable for a scoped run/test
 RETRIES="${RETRIES:-3}"                       # attempts before a transient failure aborts
 RUN_TIMEOUT="${RUN_TIMEOUT:-600}"             # seconds per backtest before it counts as a hang
 DISCOVER_MIN_YEAR="${DISCOVER_MIN_YEAR:-2005}" # earliest year the first-data probe scans
@@ -211,12 +211,18 @@ probe_run() {
 # nothing if the symbol has no data at all. Yearly probe uses December (catches data
 # that started any time that year); monthly probe uses the FULL month (catches data
 # starting after the 10th — the old probe only tested days 01–10).
+#
+# CONTRACT: this function's STDOUT carries ONLY the result line; the caller reads it via
+# command substitution. probe_run -> run_one stream the cBot's "[BOT] …" lines on stdout,
+# so every probe call here MUST be redirected to stderr (`1>&2`) — otherwise that progress
+# noise pollutes the captured value and start_idx is parsed from garbage (see the AUDUSD
+# year-0000 abort, docs/export-runner-quirks.md). All diagnostics here go to stderr too.
 discover_start() {
     local S=$1 MODE=$2 y mm MM edate found_year=""
     echo "🌑 Discovery: locating first data month for $S ($MODE)..." >&2
     for ((y=DISCOVER_MIN_YEAR; y<CURRENT_YEAR; y++)); do
         echo "  🧪 year $y" >&2
-        if probe_run "$S" "$MODE" "01/12/$y" "31/12/$y"; then found_year=$y; break; fi
+        if probe_run "$S" "$MODE" "01/12/$y" "31/12/$y" 1>&2; then found_year=$y; break; fi
     done
     if [[ -z "$found_year" ]]; then found_year=$CURRENT_YEAR; fi   # brand-new this year → check months below
     for ((mm=1; mm<=12; mm++)); do
@@ -225,7 +231,7 @@ discover_start() {
         edate="$(month_end_or_clamp "$found_year" "$mm")"
         [[ -z "$edate" ]] && break
         echo "  🧪 month $found_year-$MM" >&2
-        if probe_run "$S" "$MODE" "01/$MM/$found_year" "$edate"; then echo "$found_year $mm"; return 0; fi
+        if probe_run "$S" "$MODE" "01/$MM/$found_year" "$edate" 1>&2; then echo "$found_year $mm"; return 0; fi
     done
     return 1   # no data anywhere
 }