Improve QuickCheck shrink candidate ordering

[mizchi]

Reorders built-in QuickCheck shrink candidates from simpler to more complex values.

• Try zero before nearby numeric values
• Allow lists to shrink directly to empty
• Try None before Some candidates
• Avoid repeated zero candidates when shrinking floating-point values

A regression test with an initial value of 10000 previously exhausted all 100 shrink attempts and stopped at 9900. It now reaches the failure boundary at 5000 in one successful shrink (15 candidates examined).

Validation: moon test --target all, moon check --target all, and moon info.

[Copilot]

Pull request overview

This pull request reorders QuickCheck shrink candidates to reach simpler counterexamples faster.

Changes:

• Prioritizes zero, empty collections, and None.
• Removes duplicate floating-point zero candidates.
• Updates snapshots and adds regression coverage.

The driver regression test currently reports 27 attempts rather than the documented 15 because a second shrink pass is counted.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
quickcheck/shrink/utils.mbt	Filters duplicate floating-point zero candidates.
quickcheck/shrink/shrink.mbt	Reorders numeric shrink candidates.
quickcheck/shrink/shrink_test.mbt	Updates numeric shrink snapshots.
quickcheck/shrink/composite.mbt	Prioritizes None candidates.
quickcheck/shrink/composite_test.mbt	Updates composite snapshots.
quickcheck/shrink/collection.mbt	Allows lists to shrink directly to empty.
quickcheck/shrink/collection_test.mbt	Adds list regression coverage and updates snapshots.
quickcheck/README.mbt.md	Updates documented shrink output.
quickcheck/driver_test.mbt	Adds numeric shrinking budget regression coverage.

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