perf: add ingest and search load-test harnesses

scripts/generate_synthetic_pdfs.py builds real PDF/1.4 documents with
a hand-written xref so we can generate tens of thousands of ~2 KB
PDFs locally. Helvetica only covers latin-1, which is fine for a
load generator (throughput, not retrieval relevance); the docstring
calls this out so no one mistakes the output for a quality corpus.

scripts/load_ingest.py drives POST /ingest/folder, then polls a
hypothetical /documents/stats endpoint every poll-interval seconds
to track terminal-state progression. Writes a JSON history report so
results can be diffed between runs.

scripts/locustfile_search.py defines a SearchUser profile mixing
hybrid / lexical / semantic queries against POST /search plus a
health-check sampler. Asserts non-empty results so a "200 with
zero hits" regression surfaces as a failure rather than a green
percentile graph.

RUNBOOK gains a Load testing section with CPU/GPU SLO tables for
both axes (sustained docs/min, search latency p50/p95/p99).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/locustfile_search.py

@@ -0,0 +1,72 @@
+"""Locust load profile for the LegacyHUB hybrid search API.
+
+Run:
+
+  pip install locust
+  locust -f scripts/locustfile_search.py \
+         --host http://localhost:8000 \
+         --users 50 --spawn-rate 5 --run-time 5m
+
+Or headless with HTML report:
+
+  locust -f scripts/locustfile_search.py --host http://localhost:8000 \
+         --headless --users 100 --spawn-rate 10 --run-time 10m \
+         --html load_search.html
+"""
+
+from __future__ import annotations
+
+import random
+
+from locust import HttpUser, between, task
+
+
+QUERIES = [
+    "ГОСТ 21.501-93 рабочие чертежи",
+    "класс бетона B25",
+    "регламент технического обслуживания",
+    "контроль качества сварных соединений",
+    "схема электропитания корпус 3",
+    "журнал ремонтов узлов",
+    "правила производства земляных работ",
+    "акты приемки скрытых работ",
+    "fundament concrete grade",
+    "maintenance schedule appendix",
+]
+
+MODES = ["hybrid", "hybrid", "hybrid", "lexical", "semantic"]
+
+
+class SearchUser(HttpUser):
+    wait_time = between(0.5, 2.5)
+    api_prefix = "/api/v1"
+
+    @task(8)
+    def hybrid_search(self):
+        body = {
+            "query": random.choice(QUERIES),
+            "limit": random.choice([5, 10, 20]),
+            "filters": {
+                "document_id": None,
+                "source_path": None,
+                "block_type": None,
+                "min_ocr_confidence": None,
+            },
+            "search_mode": random.choice(MODES),
+        }
+        with self.client.post(
+            f"{self.api_prefix}/search",
+            json=body,
+            name="POST /search",
+            catch_response=True,
+        ) as res:
+            if res.status_code != 200:
+                res.failure(f"HTTP {res.status_code}: {res.text[:120]}")
+                return
+            data = res.json()
+            if not data.get("results"):
+                res.failure("empty results")
+
+    @task(1)
+    def health(self):
+        self.client.get(f"{self.api_prefix}/health", name="GET /health")