Fix runtime site pose updates after default initialization

[cvan20191]

Fixes #3029.

Summary

This fixes runtime updates to site poses when a site was initialized with a default position and/or identity quaternion.

Problem

Sites initialized with default pose values can be compiled with site_sameframe shortcuts such as mjSAMEFRAME_BODY or mjSAMEFRAME_BODYROT.

After users mutate model->site_pos or model->site_quat at runtime and call mj_forward, those compiled shortcuts can remain active. As a result, mj_local2Global may skip the current local site position or quaternion, causing data->site_xpos / data->site_xmat not to reflect the mutated model fields.

This made behavior depend on the site's initial XML pose: non-default-initialized sites updated as expected, while default-initialized sites could appear static.

Fix

mj_kinematics2 now revalidates site same-frame shortcuts against the current site pose before calling mj_local2Global.

The change is limited to site transform computation and preserves valid shortcuts where possible, including downgrading shortcuts when only part of the shortcut remains valid.

Tests

Added C++ regression coverage:

• CoreSmoothTest.SitePositionMutationUpdatesDefaultSiteXpos
• CoreSmoothTest.SiteQuatMutationUpdatesDefaultSiteXmat

These tests failed before the fix and pass after it.

Locally run:

git diff --check
# narrow gtest filter for the two new tests
# full engine_core_smooth_test
ctest --test-dir build-gcc15 -R '^CoreSmoothTest\.' --output-on-failure