cvc-elt.3.1: when an element's declaration is not nillable, the xsi:nil attribute
is forbidden entirely, whatever its value. PureXML only rejected xsi:nil="true" on
a non-nillable element, so xsi:nil="false" slipped through. ComplexValidator.nilErrors
now flags the presence of any xsi:nil on a non-nillable element via the new
nilAttributeValue helper, while a nillable element still accepts xsi:nil="false"
(explicitly not nilled, valid empty-or-content per its type).
XSTS invalid-instances-accepted 28 -> 26 (both xsi:nil-on-non-nillable cases), no
other bucket moved (valid-schemas-rejected and valid-instances-rejected held at 0
across all 14383 groups). 1452 unit tests pass.
XSD 1.0 3.3.2: an element with neither a `type` attribute nor an inline type, but
a `substitutionGroup`, takes the {type definition} of its head. PureXML defaulted
such an element to `anyType`, so an instance of it accepted any content.
`XSDParser.elementType` now resolves the head declaration through
`context.globalElements` (transitively, with a cycle guard) and uses its type.
XSTS invalid-instances-accepted 29 -> 28 (typeDef00204m: <root>Yes</root> is no
longer accepted where root substitutes a boolean head), no other bucket moved
(valid-schemas-rejected and valid-instances-rejected held at 0 across all 14383
groups). 1451 unit tests pass.
cvc-elt.4.3.2.1 requires an xsi:type to name a type DERIVED from the element's
declared type. anyType is the ur-supertype of every type, so it is never derived
from anySimpleType (its own subtype); an anyType override on an anySimpleType
element is therefore invalid. The general not-derived check stays silent on
ur-type declared types to avoid false positives, so this adds the narrow,
provably-invalid anyType-for-anySimpleType pairing only, leaving anyType valid on
an anyType-declared element (which resolves to a complex ur-type and is excluded).
XSTS invalid-instances-accepted 30 -> 29 (stZ056), no other bucket moved
(valid-schemas-rejected and valid-instances-rejected held at 0 across all 14383
groups). 1450 unit tests pass.
XSD 1.0 has no year zero (the year before 0001 is -0001), so 0000-01-01T00:00:00
and the other 0000 forms are not valid values. DateTimeParser.parseYearOnly now
rejects a zero magnitude for both signs, so negative years like -0001 still parse.
Closes the XSTS dateTime011 false negative: invalid-instances-accepted 31 -> 30,
no other bucket moved (valid-schemas-rejected and valid-instances-rejected held at
0). 1449 unit tests pass; the full XSTS suite settles at 0/43/0/30.
