Add sweep registry to prevent GC of ERROR sweeps and enable discovery

[Copilot]

ERROR state sweeps are currently lost if the user reference is deleted, may be GC'd before inspection, and reference cycles prevent cleanup of KILLED sweeps.

Changes

Registry System

• WeakValueDictionary tracks all sweep instances (allows normal GC)
• set holds strong references to ERROR sweeps (prevents GC until explicit release)
• Thread-safe with threading.Lock on all registry operations

Lifecycle Management

• mark_error() adds sweep to error hold
• kill() and clear_error() remove from error hold
• kill() calls new clear_sweep_ref() methods on runner/plotter to break cycles

Public API

from measureit import get_error_sweeps, get_all_sweeps

# ERROR sweeps remain findable even after del
s = Sweep1D(...)
s.start()
# ... error occurs
del s

# Still accessible
for sweep in get_error_sweeps():
    print(sweep.progressState.error_message)
    sweep.kill()  # Now GC-eligible

Reference Cycle Breaking

• Added clear_sweep_ref() to RunnerThread and Plotter
• Called in kill() before nulling references
• Breaks Sweep ↔ Runner ↔ Plotter cycles

Implementation Details

• _next_id counter assigns unique IDs during __init__
• get_error_sweeps() returns directly from error_hold (O(1))
• _clear_registry_for_testing() helper for test isolation
• Fully backward compatible - zero impact on existing code

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Sweep GC: ERROR sweeps should be held and findable via registry</issue_title>
<issue_description>## Problem

Currently, sweep instances in ERROR state:

1. Cannot be found if the user loses their reference
2. May be garbage collected before the user can inspect them
3. Have reference cycles (Sweep ↔ Runner ↔ Plotter) that prevent proper GC even for KILLED sweeps

Expected Behavior

1. ERROR state sweeps should be held in memory (prevent GC) until explicitly kill()ed or clear_error()ed
2. A sweep registry should allow finding sweeps by state: get_error_sweeps(), get_all_sweeps()
3. KILLED sweeps should be properly garbage collected (reference cycles should be broken)

Proposed Solution

1. Add Sweep Registry

import weakref

class BaseSweep(QObject):
    _registry = weakref.WeakValueDictionary()  # All sweeps (weak refs)
    _error_hold = set()  # Strong refs for ERROR sweeps (prevents GC)
    _next_id = 0

    @classmethod
    def get_all_sweeps(cls):
        return list(cls._registry.values())

    @classmethod
    def get_error_sweeps(cls):
        return [s for s in cls._registry.values()
                if s.progressState.state == SweepState.ERROR]

2. Hold ERROR sweeps

In mark_error():

BaseSweep._error_hold.add(self)  # Prevent GC

In kill() and clear_error():

BaseSweep._error_hold.discard(self)  # Allow GC

3. Break reference cycles

Add clear_sweep_ref() to RunnerThread and Plotter, call it in kill() before setting to None.

Files to Modify

• src/measureit/sweep/base_sweep.py
• src/measureit/_internal/runner_thread.py
• src/measureit/_internal/plotter_thread.py
• src/measureit/__init__.py

Usage After Implementation

from measureit import Sweep1D, get_error_sweeps

s = Sweep1D(...)
s.start()
# ... error occurs, reference lost ...
del s

# Can still find it:
for sweep in get_error_sweeps():
    print(sweep.progressState.error_message)
    sweep.kill()  # Now it can be GC'd

---

See full plan: .claude/plans/pure-tumbling-cherny.md</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Sweep GC: ERROR sweeps should be held and findable via registry #25

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.