fix(stress): port harnesses to v1.2 single-session API + remove WS-batch hang

Local API stress (lib.mjs / api_stress.mjs):
- setupSession now does login -> /admin/api/reset and returns sid="main".
  Drops the dead /admin/api/quizzes + /admin/api/sessions calls left over
  from the multi-quiz codex era.
- bootServer writes the fixture pool (STRESS_POOL by default) to a tmp
  file and passes QUIZ_POOL_PATH so the v1.2 server has a session at
  startup.
- happyPath: drop the post-connect lobby_update wait (race with snapshot
  dispatch) and stop double-driving the lifecycle (next() already opens
  the next question, an explicit open() afterwards is a no-op).
- cross_session: rewritten as "cookie not honored on a non-existent sid"
  since v1.2 hosts a single canonical session.

Live accuracy stress (live_accuracy.mjs):
- Per-student lobby-snapshot timeout (12s) with WS error/close rejection,
  so a stalled handshake no longer hangs Promise.all until the outer
  shell timeout (which produced the exit=124 cycles).
- Open all student WSs in parallel (mirrors what real students do); the
  batch-of-8 throttle was masking the question we wanted answered.
- Instructor WS open also bounded by a 15s race so any failure surfaces
  as actionable error text instead of a silent stall.

Bootstrap (deploy/bootstrap.sh):
- Stage 1 provisions a 2GB swap file (idempotent) with vm.swappiness=10.
  1GB-RAM ECS instances OOM-kill uvicorn under WS-burst start-of-class
  pressure; swap absorbs the spike without affecting steady state.
- Pool seeding prefers examples/demo10_pool.json over the 2-question
  example so a fresh deploy boots with a usable demo.

Pool fixture (examples/demo10_pool.json):
- 10-question generic-knowledge demo pool, gitignore exception added.

examples/demo10_pool.json

@@ -0,0 +1,87 @@
+{
+  "title": "Demo Pool: Generic Knowledge (10Q)",
+  "score_fn": "linear_decay",
+  "time_limit_default": 60,
+  "questions": [
+    {
+      "id": "d01",
+      "text": "Which of these is a programming language?",
+      "options": {"A": "HTTP", "B": "Python", "C": "TCP", "D": "DNS"},
+      "correct": "B",
+      "explanation": "Python is a general-purpose programming language; the others are network protocols."
+    },
+    {
+      "id": "d02",
+      "text": "What is 2 + 2?",
+      "options": {"A": "3", "B": "4", "C": "5", "D": "22"},
+      "correct": "B",
+      "explanation": "Basic arithmetic."
+    },
+    {
+      "id": "d03",
+      "text": "What is the capital of France?",
+      "options": {"A": "Berlin", "B": "Madrid", "C": "Paris", "D": "Rome"},
+      "correct": "C",
+      "explanation": "Paris has been the capital of France since the 10th century."
+    },
+    {
+      "id": "d04",
+      "text": "Which planet is known as the Red Planet?",
+      "options": {"A": "Venus", "B": "Mars", "C": "Jupiter", "D": "Saturn"},
+      "correct": "B",
+      "explanation": "Mars appears red because of iron-oxide dust on its surface."
+    },
+    {
+      "id": "d05",
+      "text": "Which HTTP status code means 'Not Found'?",
+      "options": {"A": "200", "B": "301", "C": "404", "D": "500"},
+      "correct": "C",
+      "explanation": "404 is the canonical client-error response for a missing resource."
+    },
+    {
+      "id": "d06",
+      "text": "What does CPU stand for?",
+      "options": {
+        "A": "Central Processing Unit",
+        "B": "Computer Personal Unit",
+        "C": "Central Performance Utility",
+        "D": "Core Programming Unit"
+      },
+      "correct": "A",
+      "explanation": "The CPU is the primary component that executes program instructions."
+    },
+    {
+      "id": "d07",
+      "text": "Which sorting algorithm has the best average-case complexity?",
+      "options": {
+        "A": "Bubble sort",
+        "B": "Selection sort",
+        "C": "Quicksort",
+        "D": "Insertion sort"
+      },
+      "correct": "C",
+      "explanation": "Quicksort averages O(n log n); the others average O(n^2)."
+    },
+    {
+      "id": "d08",
+      "text": "Approximately what is the speed of light in vacuum (m/s)?",
+      "options": {"A": "3 x 10^6", "B": "3 x 10^8", "C": "1.5 x 10^8", "D": "9.8"},
+      "correct": "B",
+      "explanation": "About 299,792,458 m/s, conventionally rounded to 3 x 10^8 m/s."
+    },
+    {
+      "id": "d09",
+      "text": "Which data structure operates strictly in Last-In-First-Out (LIFO) order?",
+      "options": {"A": "Queue", "B": "Stack", "C": "Linked list", "D": "Hash map"},
+      "correct": "B",
+      "explanation": "A stack pushes and pops from the same end."
+    },
+    {
+      "id": "d10",
+      "text": "Which of the following is NOT an operating system?",
+      "options": {"A": "Linux", "B": "Windows", "C": "Oracle", "D": "macOS"},
+      "correct": "C",
+      "explanation": "Oracle is a database management system, not an OS."
+    }
+  ]
+}