Layer A of the SP35 defense-in-depth fix. Before SP35 the dropdown
silently defaulted to '837p' and never changed when a file was dropped
on the page — uploading an 835 file routed it to /api/parse-837 which
(prior to SP35 Task 2) silently persisted an empty batch.
The change:
1. New pure helper src/lib/x12-detect.ts:
- detectKindFromText(text) reads the first ~4KB and returns the
DetectedKind ('837p' | '835' | '999' | '277ca' | 'ta1' | 'unknown')
by matching the ST01 segment (or the bare TA1 segment for the
no-ST envelope). Cheap substring scan; never invokes tokenize().
- detectKindFromFile(file) is the File-aware wrapper used by the UI.
- detectedKindToParsedBatchKind maps the DetectedKind to the kind
the Upload dropdown supports. Returns null for 999/277CA/TA1 so
the UI can surface a clean 'this file isn't supported here' hint.
2. Upload.tsx: pickFile is now async and reads the file before storing
it. If the detected kind differs from the dropdown's current value,
it switches the dropdown and toasts a hint. If the detected kind is
999/277CA/TA1 (Upload doesn't ingest those), it shows an error toast.
20 new tests in src/lib/x12-detect.test.ts cover the 6 DetectedKind
paths, the File wrapper, case-insensitivity, garbage input, the
ST*8370 false-positive guard, and the detectedKindToParsedBatchKind
mapping.
Nora