perf: add and harden synthetic scale-test suite

tests/README.md

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+# Synthetic Performance Tests
+
+This directory contains synthetic Python workloads used to stress-test Pyrefly indexing and type-solving throughput.
+
+## Structure
+
+- `perf/scale_test/`: Generated "synthetic enterprise" package with dense type annotations, deep inheritance, and import meshes.
+
+## Scale Test Goals
+
+The scale test workload is designed to apply pressure to:
+
+1. Deep recursive solving paths in inheritance hierarchies.
+2. Generic and protocol-heavy type relationships.
+3. Inter-module dependency graph construction.
+4. File-volume and line-volume indexing throughput.
+
+## Generator
+
+The scale test modules are produced by:
+
+- `perf/scale_test/generate_stress_test.py`
+
+Requirements:
+
+- Python 3.12+ available as `python3` (macOS/Linux) or `py -3` (Windows).
+- Run commands from the repository root.
+
+Optional virtual environment setup:
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+python3 -m pip install --upgrade pip
+```
+
+Windows PowerShell equivalent:
+
+```powershell
+py -3 -m venv .venv
+.\.venv\Scripts\Activate.ps1
+py -3 -m pip install --upgrade pip
+```
+
+Run from repository root:
+
+```bash
+python3 tests/perf/scale_test/generate_stress_test.py --core-only
+```
+
+Windows PowerShell equivalent:
+
+```powershell
+py -3 tests/perf/scale_test/generate_stress_test.py --core-only
+```
+
+Generate full suite (default: 120 modules, min 520 lines each):
+
+```bash
+python3 tests/perf/scale_test/generate_stress_test.py
+```
+
+Windows PowerShell equivalent:
+
+```powershell
+py -3 tests/perf/scale_test/generate_stress_test.py
+```
+
+Customize generation:
+
+```bash
+python3 tests/perf/scale_test/generate_stress_test.py \
+  --module-count 150 \
+  --min-lines 700
+```
+
+Windows PowerShell equivalent:
+
+```powershell
+py -3 tests/perf/scale_test/generate_stress_test.py --module-count 150 --min-lines 700
+```
+
+## Expected Core Outputs
+
+Running with `--core-only` emits the initial five baseline modules:
+
+- `base_types.py`
+- `data_layer.py`
+- `service_mesh.py`
+- `domain_models.py`
+- `orchestration.py`
+
+All generated files are valid Python and intentionally large to provide stable stress-testing inputs.

tests/perf/scale_test/__init__.py

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+"""Synthetic enterprise stress-test package for Pyrefly."""
+
+from . import base_types
+from . import data_layer
+from . import service_mesh
+from . import domain_models
+from . import orchestration
+
+__all__ = [
+    "base_types",
+    "data_layer",
+    "service_mesh",
+    "domain_models",
+    "orchestration"
+]