feat(data): spot_drift_detection.sh --preflight-only (Friday shell-run dry path — closes DriftDetection skip-exception)

[cipher813]

Adds a --preflight-only modifier to infrastructure/spot_drift_detection.sh, mirroring the merged alpha-engine-data #259 (spot_data_weekly.sh) / predictor #175 / backtester #224 pattern. Closes the DriftDetection skip-exception in ROADMAP "Friday shell-run — per-module dry-path activation" — the one per-module SF step still SKIPPED rather than dry-run on the Friday shell_run.

Insertion point

• PREFLIGHT_ONLY=0 modifier var, initialised before the arg-parse loop (orthogonal to RUN_MODE, set -u safe).
• --preflight-only) PREFLIGHT_ONLY=1; shift ;; added to the case loop; echo line added to the run banner.
• Guard block inserted after the smoke-only block and strictly before the # ── Full drift detection ── section (the run_remote bash -s <<DRIFT heredoc) and before the trailing aws cloudwatch put-metric-data heartbeat.

No-scan / no-write proof

monitoring.drift_detector (lives in alpha-engine-predictor, reached via the sibling-clone PYTHONPATH) is the sole code path doing any S3 get_object/put_object of the drift report or SNS publish on alert; the launcher's CloudWatch put-metric-data heartbeat trails it. The PREFLIGHT_ONLY guard exit 0s strictly before the <<DRIFT heredoc, so the drift scan, the SNS publish, the S3 put_object, and the CloudWatch emit are all statically unreachable. The preflight itself runs only BasePreflight.check_env_vars (env read) + BasePreflight.check_s3_bucket (bucket HEAD) + an importlib.import_module of the drift module (import-only — the boto3 clients + check_drift()/main() sit behind if __name__ == "__main__", which an import does not trigger). Hard invariant holds: zero external API data fetch, zero S3/CW/SNS/config mutation; early exit 0 because a passed preflight is a healthy outcome (SSM/SF report Success).

Preflight substrate reused

The drift workload binary lives in alpha-engine-predictor (no --preflight-only of its own; out of scope to modify here) and this repo's preflight.py DataPreflight modes (daily/morning_enrich/phase1/phase2) are data-collection scoped — none maps to drift. Per the canonical-lib fallback the preflight composes alpha_engine_lib.preflight.BasePreflight directly (env-vars + S3 HEAD, both read-only). No bespoke preflight scaffolding duplicated.

Verbatim flag name

--preflight-only

Tests

New tests/test_spot_drift_detection_preflight_only.py — 5 static grep / source-position assertions, mirroring tests/test_preflight_only_dry_path.py:

1. flag parses as a modifier (PREFLIGHT_ONLY=1) + PREFLIGHT_ONLY=0 init present
2. guard precedes the DRIFT section and the CloudWatch heartbeat
3. guard block exit 0s before the DRIFT section
4. no drift scan / S3 / CW / SNS write inside the block
5. canonical BasePreflight reused (env-vars + S3 HEAD) + import-only drift smoke, no scaffolding

bash -n infrastructure/spot_drift_detection.sh clean. Full data suite: 1342 passed, 1 skipped (pre-existing), 5 pre-existing warnings (unrelated daily_append.py pandas FutureWarning).

Relationship to #260

Independent: #260 touches spot_data_weekly.sh + the Lambda --dry-run keystone (a different file). The Saturday/Friday SF rewire to route the DriftDetection state at this --preflight-only flag under the Friday shell_run is a separate follow-on — no step_function.json change here.

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