fix(subsystem-edit): write stub XML on add-child operation

Do-AddChild / do_add_child added `<Subsystem>Name</Subsystem>` to the
parent's `<ChildObjects>`, but never wrote the corresponding
`Subsystems/{Parent}/Subsystems/{Name}.xml` file. Same silent-drop
pattern that bit subsystem-compile (aa93031): platform used to swallow
the missing-file reference, `-StrictLog` now surfaces it as "Файл
объекта не существует" and fails add-child on load.

Both ports now mirror the subsystem-compile fix:
- Write-ChildSubsystemStub / write_child_subsystem_stub helpers
  duplicated from subsystem-compile (per memory rule "skills are
  autonomous, duplication acceptable")
- format_version read from loaded XmlDoc root (no need to walk up
  to Configuration.xml — we already have the parent XML in memory)
- Stub creation guarded by Test-Path / os.path.exists so a pre-existing
  real child file is never clobbered

Bumped subsystem-edit.ps1 v1.1→v1.2 and subsystem-edit.py v1.1→v1.2.

Verification:
- verify-snapshots --skill subsystem-edit: 3/4 → 4/4
- runner --filter subsystem-edit (PS1): 4/4
- runner --filter subsystem-edit --runtime python: 4/4 (dual-port drift clean)

With this landed P1 from debug/snapshot-verify/NEXT-STEPS.md is fully
closed: subsystem-compile 7/7, subsystem-edit 4/4, interface-edit 4/4,
role-compile 8/8, meta-compile 30/30.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/skills/cases/subsystem-edit/snapshots/add-child/Subsystems/Продажи/Subsystems/Настройки.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<MetaDataObject xmlns="http://v8.1c.ru/8.3/MDClasses" xmlns:app="http://v8.1c.ru/8.2/managed-application/core" xmlns:cfg="http://v8.1c.ru/8.1/data/enterprise/current-config" xmlns:cmi="http://v8.1c.ru/8.2/managed-application/cmi" xmlns:ent="http://v8.1c.ru/8.1/data/enterprise" xmlns:lf="http://v8.1c.ru/8.2/managed-application/logform" xmlns:style="http://v8.1c.ru/8.1/data/ui/style" xmlns:sys="http://v8.1c.ru/8.1/data/ui/fonts/system" xmlns:v8="http://v8.1c.ru/8.1/data/core" xmlns:v8ui="http://v8.1c.ru/8.1/data/ui" xmlns:web="http://v8.1c.ru/8.1/data/ui/colors/web" xmlns:win="http://v8.1c.ru/8.1/data/ui/colors/windows" xmlns:xen="http://v8.1c.ru/8.3/xcf/enums" xmlns:xpr="http://v8.1c.ru/8.3/xcf/predef" xmlns:xr="http://v8.1c.ru/8.3/xcf/readable" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.17">
+	<Subsystem uuid="UUID-001">
+		<Properties>
+			<Name>Настройки</Name>
+			<Synonym/>
+			<Comment/>
+			<IncludeHelpInContents>true</IncludeHelpInContents>
+			<IncludeInCommandInterface>true</IncludeInCommandInterface>
+			<UseOneCommand>false</UseOneCommand>
+			<Explanation/>
+			<Picture/>
+			<Content/>
+		</Properties>
+		<ChildObjects/>
+	</Subsystem>
+</MetaDataObject>