[siracozmen01]

Problem

m2Body_SetSleepThreshold's header says the change applies from the next step and a sleeping body stays asleep. No test sets a threshold on a sleeping body, so a wake on the call, or a threshold lost while asleep, would pass every suite (#347).

Change

test/test_islands.c gains TestThresholdOnASleepingBody:

• A box glides at 0.03 m/s with no gravity, and falls asleep at the default 0.05 m/s.
• It is given a threshold of 0.02 m/s while asleep. It is still asleep on the call and 30 steps later.
• It is then woken at 0.03 m/s with m2Body_SetLinearVelocity, and stays awake for a second: the threshold set while asleep governs.
• As the near miss, the same run with the default set again also stays asleep, and once woken sleeps again.

A test-only change.

Verification

• test_islands passes.
• Mutants, each built into a scratch library and run against test_islands:

Mutant	Fails
the threshold change wakes the body	both checks: the body is awake after the call
the threshold change keeps the old value	the existing readbacks, and the 0.02 m/s check: the woken body sleeps again

• tools/check.sh changed passes. The changed file compiles clean with gcc-14 and clang-20 under the strict flags.
• test_golden prints the 13 lines of test/hashes.txt unchanged in the scalar (linux-clang-20-asan) and AVX2 (linux-clang-20) presets.

Closes #347

[siracozmen01]

Review at 997ad36: ready. Read, not run.

The keep-old-value mutant fails on the threshold's effect, not only on the awake flag.

• After the call, the body is woken at 0.03 m/s and run for a second, and the check needs it still awake: !GlideAfterThreshold(0.02f, ...).
• 0.03 is above the new 0.02 and below the old 0.05. So if the old value is kept, the body sleeps again within that second (the run before the call shows it does at 0.05), and the first check fails. That is the right reason.
• The near miss (0.05 set again, the same run) has to sleep again. This shows the only thing separating the two runs is the threshold, not the wake by m2Body_SetLinearVelocity or the time allowed.

The wake mutant. asleep is read straight after the call and again 30 steps later. Both checks need stillAsleep, so a wake on the call fails both.

Each assertion can fail, nothing is weakened or removed, and the body has Problem, Change and Verification.