FP lane gaining missed-smoke stays annotated with no boxes, and exports

An FP lane sitting at annotated can gain missed-smoke — and therefore start needing localization — without changing stage, so neither the localization guard nor the FP→smoke demotion fires. The lane ends up at annotated, needing localization, with zero boxes: on /localize/done and in /export/alerts.

This is the state #346 set out to prevent. PR #352 closed every path into annotated; this one never crosses that boundary.

Reproduced

SEEDED:      [('annotated', {'annotation': []})]   # FP lane's empty placeholder
PATCH {"has_missed_smoke": true}
PATCH status: 200
RESULT stage: annotated
RESULT has_missed_smoke: True
IN /localize/done: True
IN /export/alerts: True

Why both guards miss it

• The localization guard (sequence_annotations.py, assert_localization_complete) requires the stage to be changing into annotated. Here target_processing_stage == existing.processing_stage == ANNOTATED, so it is skipped. Widening it would not help either: the FP exit already wrote empty annotated-stage detection annotations via auto_create_detection_annotations, so assert_localization_complete would pass on those rows.
• The FP→smoke promotion (is_fp_promotion) requires target_processing_stage == SEQ_ANNOTATION_DONE. A flags-only PATCH leaves the stage alone, so the demotion — which would delete those empty rows and re-arm auto-annotate — never runs.

Reachable from the UI, not just the API

frontend/src/pages/LocalizeAlertPage.tsx sends exactly this PATCH from the missed-smoke row:

apiClient.updateSequenceAnnotation(annotationId, { has_missed_smoke: value })

and missedSmokeAnnotationId falls back to "the first lane that has an annotation at all", which can be the FP lane. The row renders in done mode.

Suggested fix

Key is_fp_promotion on the flag transition rather than the target stage: a lane at annotated that did not previously need localization but does after this edit should demote to seq_annotation_done, whatever stage the payload names. That is the intent issue #275 already established — the current predicate just happens to detect it only when the client also sends the new stage.

Worth confirming as part of that work: a flags-only PATCH would then change processing_stage without the client asking, which callers should expect.

Found by review of #352; not a regression (the same gap existed before it, via the existing == SEQ_ANNOTATION_DONE form of the guard).

Start with is_fp_promotion and the localization guard in sequence_annotations.py, then trace the missed-smoke PATCH from frontend/src/pages/LocalizeAlertPage.tsx. Reproduce the flags-only update described in the issue and verify that a lane newly needing localization is demoted from annotated to seq_annotation_done, avoiding the zero-box localization and export states.