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/empty-param-values.json

@@ -0,0 +1,27 @@
+{
+  "name": "Параметры с пустыми значениями (все типы, разные sentinel-формы)",
+  "params": { "outputPath": "Template.xml" },
+  "input": {
+    "dataSets": [{
+      "name": "Основной",
+      "query": "ВЫБРАТЬ 1 КАК Поле1",
+      "fields": ["Поле1: число(1,0)"]
+    }],
+    "parameters": [
+      "Параметр1",
+      "Параметр2: string =",
+      "ПараметрСписок: EnumRef.СтатусТеста @valueList = _",
+      "ПараметрСсылка: CatalogRef.ПлоскийПростой",
+      "ПараметрДата: date = null",
+      { "name": "ПараметрЧисло", "type": "decimal", "value": null },
+      "ПараметрБулево: boolean = ",
+      "ПараметрСтандартныйПериод: StandardPeriod = _",
+      { "name": "ПараметрТипНеЗадан", "value": null },
+      "ПараметрСписокСтрок: string @valueList"
+    ]
+  },
+  "validatePath": "Template.xml",
+  "expect": {
+    "files": ["Template.xml"]
+  }
+}