fix(skd-compile): Designer-compatible empty parameter values

Centralized empty-value handling: shorthand `=`, `= _`, `= null` and
object-form `value: null` / `""` now serialize per type, matching what 1C
Designer writes:
- ref / no-type → <value xsi:nil="true"/>
- string → <value xsi:type="xs:string"/>
- date/decimal/boolean → typed zero (0001-01-01 / 0 / false)
- StandardPeriod → Custom variant with zero dates
- @valueList → omit <value> entirely

Closes BUG-1 (StandardPeriod @autoDates) and BUG-2 (CatalogRef.X = _
producing invalid <value>_</value>) reported by titan team. New helpers
Test-EmptyValue / Emit-EmptyValue (ps1) and is_empty_value /
emit_empty_value (py) shared by Emit-ParamValue, availableValues loop,
and explicit dataParameters emit. Shorthand regex .+ → .* so trailing
`=` parses as empty.

Reference: upload/erf/ПроверкаЭкранирования (live Designer dump).
New test case empty-param-values covers all 10 type×sentinel combos;
3 existing snapshots regenerated to include the now-correct <value>
tags.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/skills/cases/skd-compile/snapshots/auto-data-parameters/Template.xml

@@ -77,6 +77,11 @@
 		<valueType>
 			<v8:Type>v8:StandardPeriod</v8:Type>
 		</valueType>
+		<value xsi:type="v8:StandardPeriod">
+			<v8:variant xsi:type="v8:StandardPeriodVariant">Custom</v8:variant>
+			<v8:startDate>0001-01-01T00:00:00</v8:startDate>
+			<v8:endDate>0001-01-01T00:00:00</v8:endDate>
+		</value>
 	</parameter>
 	<parameter>
 		<name>Флаг</name>
@@ -129,6 +134,7 @@
 				<v8:AllowedLength>Variable</v8:AllowedLength>
 			</v8:StringQualifiers>
 		</valueType>
+		<value xsi:type="xs:string"/>
 	</parameter>
 	<parameter>
 		<name>Валюта</name>