[wip] Add experimental qcodes.measure_v2 package (tracer bullet)

[Jens Hedegaard Nielsen (jenshnielsen)]

Introduce a parallel, async-capable measurement API for QCoDeS that decouples plan description from execution. Plans are Python generators yielding typed messages (Set, Read, Sleep, Emit, OpenRun, CloseRun); a MeasurementEngine drives them on a dedicated worker thread and publishes events (RunStarted, RowEmitted, RunStopped) to pluggable sinks via a publisher thread with a bounded queue.

Key properties of the tracer-bullet implementation:

• Non-blocking submission via engine.submit(...) returning a RunHandle with futures for the dataset and final result.
• Cancellation via it.throw(CancelRequested) at the next yield point; plan try/finally blocks always run for safe cleanup. scan_1d guarantees a ramp-to-zero of the swept parameter on any exit path.
• Cancel flag re-checked between .send() and dispatch to prevent one extra Set/Read leaking to hardware after cancellation.
• handle.wait() returning guarantees data is durably committed: the publisher thread completes futures only after all sinks have processed RunStopped.
• SqliteSink reuses existing DataSaver/Measurement infrastructure; opens its own SQLite connection on the publisher thread to satisfy check_same_thread=True. ~50 lines of integration code.
• MemorySink for tests and lightweight in-memory consumption.

Public surface:

import qcodes as qc ds = qc.measure_v2.scan(LinSweep(g, 0, 1, 11), measure=[i])

Advanced users construct an explicit MeasurementEngine and submit plans built via run(...) + plan-builders (scan_1d, or custom generators).

Tracer scope is deliberately small: 1D scans of scalar parameters with the default SQLite sink. Multi-dim scans, Call/Describe messages, parallel reads via underlying_instrument grouping, queueing, pause/resume, live-plot/progress sinks, and a dond shim are P0-P2 for the v1 release. See src/qcodes/measure_v2/DESIGN.md for the full architecture.

50 tests across L1 (message-stream), L2 (engine + MemorySink), and L3 (engine + SqliteSink) levels. All three end-to-end acceptance scenarios pass.

[codecov]

Codecov Report

❌ Patch coverage is 90.83045% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.41%. Comparing base (0a13dca) to head (c09ad89).
⚠️ Report is 477 commits behind head on main.

Files with missing lines	Patch %	Lines
src/qcodes/measure_v2/engine.py	88.59%	35 Missing ⚠️
src/qcodes/measure_v2/convenience.py	71.87%	9 Missing ⚠️
src/qcodes/measure_v2/sinks/sqlite.py	87.14%	9 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #8116 +/- ##
==========================================
- Coverage 70.80% 70.41% -0.39% 
==========================================
Files 302 312 +10 Lines 31911 32474 +563 ==========================================
+ Hits 22593 22865 +272 - Misses 9318 9609 +291 

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