Generate the MJCF grammar table and enforce its constraints.

The hand-written MJCF[] table in xml_native_reader.cc is replaced by
mjcf_table.inc, emitted from mjcf.schema by generate_mjcf_table.py and
checked for freshness by doc_test. nMJCF is now self-sizing. The two
tables are identical as trees of (tag, cardinality, attribute-set);
within-row attribute order changes where the schema factors shared
groups and projects default-context rows, and top-level rows follow
the schema's dependency order -- neither affects validation, which is
set-based, nor XMLschema.rst, whose generator orders sections itself
(regenerated here, reading the .inc instead of the reader source).

The schema's constraint declarations become enforcement: the emitter
writes a companion MJCF_constraints[] array (row-indexed into MJCF[]),
and mjXSchema::Check evaluates each element's constraints after its
attribute check, with uniform messages derived from the declaration:
"at most one of 'fovy', 'sensorsize' can be specified", "attributes
'reftype', 'refname' must be specified together", and so on.
Multi-attribute bundles render as ('site1', 'site2').

Fifteen hand-written co-occurrence checks across fourteen elements are
deleted -- connect/weld semantics mixing and completeness, the actuator
transmission mutex, camera fovy/sensorsize, light directional/type,
inertial fullinertia-versus-orientation, rangefinder and the distance
family, contact's matching criteria, user-sensor pairing, the frame
family's reftype/refname, size memory exclusivities, mesh builtin
exclusions, and attach body/frame (newly declared). Tests assert the
uniform messages.

Two findings along the way: sensorsize-requires-resolution is a
value-level compiler rule (positive resolution), not a presence rule --
a presence constraint would be wrong and is not declared; and Size()'s
nstack/njmax range checks tested the spec value before assignment, so
they never validated the parsed value -- now they do.

Verified by compiling all 81 models in the model/ corpus.

PiperOrigin-RevId: 958064622
Change-Id: I802cf5c0aee08a62926e36a281320ff9e34c0668

test/engine/engine_sensor_test.cc

@@ -698,9 +698,9 @@ TEST_F(SensorTest, BadContact) {
       {"geom1='sphere1' geom2='sphere2' num='-3'",
        "'num' must be positive in sensor"},
       {"geom1='sphere1' geom2='sphere2' site='site'",
-       "at most one of (geom1, body1, subtree1, site) can be specified"},
+       "at most one of 'geom1', 'body1', 'subtree1', 'site' can be specified"},
       {"geom2='sphere1' body2='body'",
-       "at most one of (geom2, body2, subtree2) can be specified"},
+       "at most one of 'geom2', 'body2', 'subtree2' can be specified"},
   };
 
   for (const auto& test : test_cases) {