Import docs: adoption playbook + superseded-mappers note + loader status

Follow-up to the mapper retirement (the new docs missed the prior commit's
staging). Adds docs/IMPORT-ADOPTION.md (two-layer import story + stage/crosswalk
guidance), scripts/migration/README.md (dir superseded, points at the API +
loader), and updates the WJ loader README to complete status.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/site_imports/wjf/README.md

@@ -33,13 +33,16 @@ resolved decisions, and remaining stages.
 
 ## Status
 
-- **Implemented + verified idempotent:** `reference` (vendors, businessunits,
-  operatingsystems), `employees` (directory bulk upsert; photos deferred).
-- **TODO stages:** `models`, `applications`, `assets` (the hub - fan machines
-  out by type, persist the machineid->assetid crosswalk), `dependents`
-  (installs, warranties, notifications, KB), `network` (+ subnets/VLANs), `usb`
-  (cmmc device + checkinout pairing), `verify`.
+Complete - all 15 stages built and verified end-to-end against a fresh scratch
+target (zero endpoint errors): `reference`, `employees`, `catalog`, `assets`
+(the hub + machineid->assetid crosswalk), `locations`, `printers`,
+`communications`, `applications` (+ installs), `warranties`, `notifications`,
+`knowledgebase`, `relationships`, `subnets`, `usb`, `verify`.
 
-The harness (PAT auth, import-mode, id-map persistence, endpoint error capture)
-is proven; the remaining stages are additional `stage_*` functions in `run.py`
-following the same shape.
+Last full run: 983 assets (computer 663, machine 76, network 58, measuring-tool
+136, printer 50), 24 locations, 415 employees, 850 installs, 461 primary IPs,
+464 warranties, 261 notifications, 341 KB, 93 relationships, 37 subnets, 18 USB
+devices + 232 events. UI spot-check passed (computer list + shop-floor map).
+
+Known follow-ups: general asset `locationid` is null (no source column outside
+printers); the ~146 duplicate machinenumbers are first-wins-skipped by decision.