test(e2e): make Playwright BASE_URL-overridable + record Phase 2 e2e baseline

- playwright.config.ts: honor BASE_URL env (and skip the auto-started webServer when
  set) so a server booted from a specific worktree on a non-default port can be tested
  without fighting over :3333.
- skill test-recipes.md: record the recipe for running e2e from a non-default worktree
  (in-process test server + reseed helper) and the measured baseline on the merged
  hx-select reference: swap-doctrine 6/6 green; transaction-edit.spec.ts has a
  pre-existing Shared-Location save failure that masks 7 via serial mode; full suite
  30 pass / 2 fail / 7 skip. Gate for the refactor = swap spec + REPL pure-fn checks.

playwright.config.ts

@@ -1,5 +1,11 @@
 import { defineConfig, devices } from '@playwright/test';
 
+// Allow pointing the suite at an already-running test server (e.g. one booted from a
+// specific worktree on a non-default port) via BASE_URL. When BASE_URL is set we skip
+// the auto-started webServer entirely, so parallel worktrees don't fight over :3333.
+const baseURL = process.env.BASE_URL ?? 'http://localhost:3333';
+const useExternalServer = !!process.env.BASE_URL;
+
 export default defineConfig({
   testDir: './e2e',
   fullyParallel: true,
@@ -8,15 +14,17 @@ export default defineConfig({
   workers: process.env.CI ? 1 : undefined,
   reporter: 'html',
   use: {
-    baseURL: 'http://localhost:3333',
+    baseURL,
     trace: 'on-first-retry',
   },
-  webServer: {
-    command: 'lein run -m auto-ap.test-server',
-    url: 'http://localhost:3333/test-info',
-    reuseExistingServer: !process.env.CI,
-    timeout: 120000,
-  },
+  webServer: useExternalServer
+    ? undefined
+    : {
+        command: 'lein run -m auto-ap.test-server',
+        url: 'http://localhost:3333/test-info',
+        reuseExistingServer: !process.env.CI,
+        timeout: 120000,
+      },
   projects: [
     {
       name: 'chromium',