Add CLAUDE.md with repo guidance

Add CLAUDE.md to provide guidance for Claude Code and contributors. The file documents project identity (MeasureIt/qmeasure), build & development commands, testing conventions, architecture (sweep hierarchy, concurrency guard, threading model, package layout), data directory behavior, sweep timing constraints, code style, and known issues. Intended to help code-assistants and developers understand how to run, test, and extend the project.

Author: caidish

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Identity
+
+MeasureIt is a QCoDeS-based measurement software for condensed matter physics experiments. It is published on PyPI as `qmeasure` but imported as `measureit`. The repository name is `MeasureIt`.
+
+## Build & Development Commands
+
+```bash
+# Install (editable with all dev deps)
+pip install -e ".[dev,docs,jupyter]"
+
+# Run all tests with coverage
+pytest
+
+# Run specific test categories
+pytest tests/unit -v
+pytest tests/integration -v
+pytest tests/e2e -v
+pytest -m "not slow"
+
+# Run a single test file
+pytest tests/unit/test_sweep1d.py -v
+
+# Lint and format
+make format          # ruff format + ruff check --fix
+make lint            # ruff format --check + ruff check + mypy
+
+# Build docs
+make docs            # outputs to docs/source/_build/html/
+```
+
+Tests use `QT_QPA_PLATFORM=offscreen` (set in `tests/conftest.py`). Qt/pytest-qt integration tests are skipped in CI but should be run locally. Each test gets an isolated `MEASUREIT_HOME` via the `temp_measureit_home` fixture.
+
+## Architecture
+
+### Sweep Hierarchy
+
+`BaseSweep` (in `src/measureit/sweep/base_sweep.py`) is the central class. It owns:
+- A `RunnerThread` (QThread in `_internal/runner_thread.py`) for data acquisition
+- A `Plotter` (QObject in `_internal/plotter_thread.py`) for real-time visualization
+- `ProgressState` / `SweepState` for status tracking
+- Parameter following, QCoDeS `Measurement` creation, and save/plot lifecycle
+
+Concrete sweep types: `Sweep0D` (time-based), `Sweep1D` (single param), `Sweep2D` (dual param, composes an inner `Sweep1D`), `SimulSweep`, `SweepIPS`, `GateLeakage`, `Sweep1D_listening`.
+
+### Concurrency Guard
+
+A module-level `WeakSet` (`_ACTIVE_SWEEPS`) enforces that only one non-queued sweep runs at a time. Inner sweeps and sweeps sharing a parent chain are considered "related" and allowed. `SweepQueue` bypasses this guard. `start_force()` kills unrelated active sweeps before starting.
+
+### Threading Model
+
+All threading uses PyQt5 `QThread` + signals/slots, **not** Python `threading.Thread`. This is required even in headless/Jupyter usage because the sweep loop (`RunnerThread.run()`) communicates data points back via `pyqtSignal`. The `conftest.py` provides a session-scoped `qapp` fixture and per-test `cleanup_qt_threads` to prevent segfaults.
+
+### Package Layout
+
+```
+src/measureit/
+    sweep/              # Sweep classes (base_sweep, sweep0d/1d/2d, simul_sweep, etc.)
+    _internal/          # RunnerThread, Plotter (not public API)
+    tools/              # sweep_queue, util (init_database, safe_get/set), safe_ramp, tracking
+    visualization/      # heatmap_thread, helper (pyqtgraph-based)
+    legacy/             # Old matplotlib-based plotter/heatmap threads
+    Drivers/            # QCoDeS instrument drivers for lab hardware
+    config.py           # Data directory resolution (MEASUREIT_HOME / platformdirs)
+    logging_utils.py    # Sweep file logging + Jupyter notebook log handler
+    _deprecation.py     # FutureWarning shims for old import paths
+```
+
+Top-level shim modules (`base_sweep.py`, `sweep0d.py`, etc. at `src/measureit/`) re-export from `sweep/` with deprecation warnings. New code should import from `measureit.sweep.*` or the top-level `measureit` namespace.
+
+### Data Directory
+
+Priority: `set_data_dir()` > `MEASUREIT_HOME` env > legacy `MeasureItHome` env > `platformdirs` default. Subdirectories (`Databases`, `logs`, `cfg`, `Origin Files`) are created lazily on first access via `get_path()`.
+
+### Sweep Timing Constraints
+
+`inter_delay` minimum: 0.01s (10ms). `outer_delay` minimum: 0.1s (100ms). Values below these raise `ValueError`.
+
+## Testing Conventions
+
+- Mock instruments use `qcodes.parameters.Parameter` subclasses (see `conftest.py::mock_parameter`)
+- `fast_sweep_kwargs` fixture provides `inter_delay=0.01, save_data=False, plot_data=False, suppress_output=True`
+- `conftest.py` auto-clears QCoDeS instrument registry and `_ACTIVE_SWEEPS` between tests
+- Pandas is stubbed in `conftest.py` to avoid binary-compat issues in CI
+
+## Code Style
+
+- Ruff (line-length 88, Google docstrings, `py38` target)
+- Mypy with strict settings (configured in `pyproject.toml`)
+- `*_ui.py` files are auto-generated and excluded from all linting
+
+## Known Issues
+
+- `ipykernel` 7.0.x breaks Qt event loop; pinned to `>=6.29,!=7.*`
+- pytest-qt can cause macOS segfaults during teardown; `conftest.py` includes extensive guards