feat(sp35): Upload page auto-detects 837P vs 835 from file content

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.

src/pages/Upload.tsx

@@ -33,6 +33,10 @@ import { StatPill, ValidationDot } from "@/components/ClaimCard/shared";
 import { api, ApiError, type BatchSummary, type ParseProgress } from "@/lib/api";
 import { downloadBlob } from "@/lib/download";
 import { fmt, toNum } from "@/lib/format";
+import {
+  detectKindFromFile,
+  detectedKindToParsedBatchKind,
+} from "@/lib/x12-detect";
 import { useAppStore } from "@/store";
 import { useParse } from "@/hooks/useParse";
 import { useBatchExport } from "@/hooks/useBatchExport";
@@ -437,9 +441,37 @@ export function Upload() {
   // hook call above. Keeping the comment as a breadcrumb for future
   // readers who grep "Auto-select".)
 
-  function pickFile(f: File | null) {
+  async function pickFile(f: File | null) {
     setFile(f);
     setStream({ items: [], expectedTotal: null, passed: 0, failed: 0 });
+    if (!f) return;
+    // SP35: auto-detect the kind from the file's first few KB and switch
+    // the dropdown before the user hits Parse. This is layer A of the
+    // defense-in-depth fix (layer B is the server-side envelope guard
+    // in cyclone.api). Before SP35 the dropdown defaulted to "837p"
+    // silently — dropping an 835 file on the page routed it to
+    // /api/parse-837 and produced a bogus empty batch. Detection runs
+    // asynchronously on a 4KB slice; if it can't determine the kind
+    // (unknown file, read error) we keep the user's current selection
+    // and the server guard will surface a clean 400 if it's wrong.
+    const detected = await detectKindFromFile(f);
+    const matched = detectedKindToParsedBatchKind(detected);
+    if (matched && matched !== kind) {
+      setKind(matched);
+      setPayer(matched === "837p" ? PAYERS_837[0]!.value : PAYERS_835[0]!.value);
+      toast.message(
+        `Detected ${matched === "837p" ? "837P" : "835"} file — switched the dropdown.`,
+        { description: f.name },
+      );
+    } else if (detected === "999" || detected === "277ca" || detected === "ta1") {
+      // The Upload page only supports 837P and 835; for the other X12
+      // kinds (which have their own endpoints), surface a hint instead
+      // of silently leaving the dropdown on whatever the user picked.
+      toast.error(
+        `Detected ${detected.toUpperCase()} file — the Upload page only accepts 837P or 835.`,
+        { description: "Use the matching endpoint from the History tab or the API." },
+      );
+    }
   }
 
   async function onParse() {