Add explicit recentering stand-down coverage for pinned/frozen bodies

[Copilot]

RecentreOnOrigin is documented to stand down entirely when any body is pinned or frozen, but RecentringTests did not exercise that branch. A refactor that only skipped pinned/frozen bodies during shift application could preserve incidental tests while violating translation invariants.

• Coverage gap closed: pinned/frozen stand-down behavior

  • Added focused tests in RecentringTests for both IsPinned and IsFrozen.
  • Each test settles a corpus graph, moves it off-origin, anchors one body, then steps again and verifies:
    • the anchored body position is unchanged, and
    • the drawn-box center is unchanged (i.e., no partial recentering of unanchored bodies).

• Test isolation for mechanism-level assertion

  • Added a small helper that disables non-recentring forces (repulsion, springs, directional/flattening/untwist terms, gravity, overlap pass) so assertions are specific to recentering stand-down semantics.

core.Bodies[0].IsPinned = 1; // equivalent frozen-path test uses IsFrozen
Vec2D centreBefore = DrawnCentre(core);

for (int frame = 0; frame < 120; frame++)
{
	core.Step(1.0 / 60.0);
}

Vec2D centreAfter = DrawnCentre(core);
Assert.AreEqual(centreBefore.X, centreAfter.X, 0.0001);
Assert.AreEqual(centreBefore.Y, centreAfter.Y, 0.0001);

• Fixes No test covers the recentring pass standing down when a body is pinned or frozen #392

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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