feat(scrape): offline prove smoke and snapshot-only mode

Add --snapshot-only and --compare-snapshots to prove-incremental-append,
fix EXIT trap status, wire prove smoke into CI, and document LFG closure plan.

scripts/tests/prove-incremental-append-smoke.sh

@@ -0,0 +1,78 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+REPO_ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd -P)
+PROVE="$REPO_ROOT/scripts/prove-incremental-append.sh"
+TMP_DIR=$(mktemp -d "${TMPDIR:-/tmp}/dce-prove-smoke.XXXXXX")
+ARCHIVE_ROOT="$TMP_DIR/archive"
+CONFIG_PATH="$TMP_DIR/config.json"
+BEFORE="$TMP_DIR/before.tsv"
+AFTER="$TMP_DIR/after.tsv"
+
+cleanup() {
+  rm -rf "$TMP_DIR"
+}
+trap cleanup EXIT
+
+mkdir -p "$ARCHIVE_ROOT/demo"
+
+cat >"$ARCHIVE_ROOT/demo/Guild - general [111111111111111111].json" <<'JSON'
+{
+  "guild": {"id": "1", "name": "Guild"},
+  "channel": {"id": "111111111111111111", "name": "general"},
+  "messages": [
+    {"id": "1", "timestamp": "2020-01-01T00:00:00+00:00", "type": "Default", "content": "one"}
+  ],
+  "messageCount": 1
+}
+JSON
+
+printf '{"messages":[\n' >"$ARCHIVE_ROOT/demo/truncated [222222222222222222].json"
+
+cat >"$CONFIG_PATH" <<JSON
+{
+  "archive_root": "$ARCHIVE_ROOT",
+  "targets": [
+    {
+      "name": "demo",
+      "kind": "guild",
+      "output_dir": "$ARCHIVE_ROOT/demo",
+      "enabled": true
+    }
+  ]
+}
+JSON
+
+DCE_PRIMARY_CONFIG="$CONFIG_PATH" "$PROVE" --target demo --snapshot-only --snapshot-file "$BEFORE"
+
+if ! grep -q '111111111111111111' "$BEFORE"; then
+  printf 'ERROR: snapshot missing valid channel archive\n' >&2
+  exit 1
+fi
+if grep -q '222222222222222222' "$BEFORE"; then
+  printf 'ERROR: invalid JSON file should be skipped in snapshot\n' >&2
+  exit 1
+fi
+
+cat >"$ARCHIVE_ROOT/demo/Guild - general [111111111111111111].json" <<'JSON'
+{
+  "guild": {"id": "1", "name": "Guild"},
+  "channel": {"id": "111111111111111111", "name": "general"},
+  "messages": [
+    {"id": "1", "timestamp": "2020-01-01T00:00:00+00:00", "type": "Default", "content": "one"},
+    {"id": "2", "timestamp": "2020-01-02T00:00:00+00:00", "type": "Default", "content": "two"}
+  ],
+  "messageCount": 2
+}
+JSON
+
+DCE_PRIMARY_CONFIG="$CONFIG_PATH" "$PROVE" --target demo --snapshot-only --snapshot-file "$AFTER"
+"$PROVE" --compare-snapshots "$BEFORE" "$AFTER"
+
+if "$PROVE" --compare-snapshots "$AFTER" "$BEFORE" 2>/dev/null; then
+  printf 'ERROR: shrink comparison should have failed\n' >&2
+  exit 1
+fi
+
+printf 'prove-incremental-append-smoke: ok\n'