Change mjtSize from uint64_t to int64_t.

This change makes `mjtSize` a signed, rather than unsigned 64-bit integer. While sizes are non-negative, the type promotion rules for unsigned integers can lead to unituitive downstream bugs, and 8 billion terabytes are enough bytes.

PiperOrigin-RevId: 834292964
Change-Id: Iea9c47c2a5d80fde63851a543d59ed0549db0f01

doc/APIreference/APItypes.rst

@@ -100,7 +100,7 @@ Size type used to represent buffer sizes.
 
 .. code-block:: C
 
-   typedef uint64_t mjtSize;
+   typedef int64_t mjtSize;
 
 
 .. _tyEnums:

doc/changelog.rst

@@ -29,7 +29,11 @@ General
   - ``sidesite -> MjsSite|None``
   - ``coef -> real``
   - ``divisor -> real``
-- Non-breaking API change: :ref:`mj_sizeModel` now returns an :ref:`mjtSize` rather than an ``int``.
+
+- Non-breaking ABI changes:
+
+  - :ref:`mjtSize` is now defined as ``int64_t`` rather than ``uint64_t`` to avoid future type-promotion bugs.
+  - :ref:`mj_sizeModel` now returns an :ref:`mjtSize` rather than an ``int``.
 
 MJX
 ^^^