refactor(import): move the CSV import writer out of its route

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
-  "count": 372,
-  "total": 129866,
+  "count": 371,
+  "total": 129459,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -22,7 +22,6 @@
     "app/api/encounters/[id]/route.ts :: apiHandler(arg) #2": 269,
     "app/api/environment/route.ts :: apiHandler(arg)": 201,
     "app/api/export/health-record/route.ts :: apiHandler(arg)": 241,
-    "app/api/import/csv/route.ts :: apiHandler(arg)": 407,
     "app/api/ingest/medication/route.ts :: apiHandler(arg)": 267,
     "app/api/insights/cards/route.ts :: apiHandler(arg)": 211,
     "app/api/insights/comprehensive/route.ts :: buildComprehensiveResponse": 505,

src/__tests__/measurement-write-plausibility-guard.test.ts

@@ -91,7 +91,7 @@ const WRITERS: Record<string, Disposition> = {
   // insert after `validateMeasurementRange`; day totals through the gated
   // reconcile seam.
   "lib/import/health-connect/import.ts": { kind: "validator" },
-  "app/api/import/csv/route.ts": {
+  "lib/import/csv-write.ts": {
     kind: "upstream",
     validatedIn: "lib/import/csv-measurements.ts",
   },

src/__tests__/measurement-write-tail-guard.test.ts

@@ -52,7 +52,7 @@ const PROVIDER_TAIL = "runs afterProviderWrite";
  * fails too).
  */
 const WRITE_SITES: Record<string, string> = {
-  "app/api/import/csv/route.ts": TAIL,
+  "lib/import/csv-write.ts": TAIL,
   "lib/import/json/measurements.ts": TAIL_VIA("lib/import/json/run.ts"),
   "app/api/measurements/[id]/route.ts": TAIL,
   "app/api/measurements/bulk-delete/route.ts": TAIL,