test(subsystem-compile): cover bottom-up -Parent flow; hide children shortcut

The bottom-up flow (compile child with -Parent pointing at parent's
XML — skill creates the real child file AND registers it in parent's
ChildObjects) has been the documented canonical way to build nested
subsystems since forever. It's in SKILL.md Примеры:58 and implemented
in subsystem-compile.ps1:430-506. But zero test cases exercised it —
all 7 pre-existing cases used the top-down `children: [...]` shortcut
that aa93031 made honest with stubs.

Two problems with the status quo:

1. A model reading SKILL.md saw `"children": ["ДочерняяА", "ДочерняяБ"]`
   right in the main JSON-definition example and took it as the
   canonical way to create nested structure. It's a trap — the
   shortcut creates placeholder stubs with empty Synonym/Content that
   the model almost never actually wants. The natural flow (one
   subsystem-compile call per real subsystem) wasn't visible where
   the model looks first.

2. The canonical flow had no test safety net — nothing caught regressions
   in the register-in-parent code path (lines 430-506).

Fix, minimal surface:

- SKILL.md: remove `"children": [...]` from the JSON-definition example.
  Leave the `-Parent` example in the Примеры section (already there).
  The children field stays fully supported in the scripts (aa93031 stub
  behavior unchanged) for legacy JSON — just not advertised.

- New test case `nested-parent.json`: preRun compiles "Продажи" parent,
  main run compiles "Настройки" child with `-Parent Subsystems/Продажи.xml`.
  Verifies the real bottom-up flow: snapshot shows full child file with
  real Synonym/Explanation AND parent's `<ChildObjects>` updated to
  reference the child. verify-snapshots confirms platform accepts it.

- Runner plumbing: `_skill.json` gains `{ "flag": "-Parent", "from":
  "workPath", "field": "parent", "optional": true }`. Required extending
  both `tests/skills/runner.mjs` and `tests/skills/verify-snapshots.mjs`
  (they each have their own copy of buildArgs) to support `optional: true`
  on workPath mappings — otherwise existing cases without params.parent
  would get the flag pushed with an empty value.

Verification:
- runner --filter subsystem-compile (PS1): 8/8 (was 7/7 +1)
- runner --filter subsystem-compile --runtime python: 8/8 (dual-port clean)
- verify-snapshots --skill subsystem-compile: 8/8

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

tests/skills/cases/subsystem-compile/snapshots/nested-parent/Languages/Русский.xml

@@ -0,0 +1,16 @@
+<?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">
+	<Language uuid="UUID-001">
+		<Properties>
+			<Name>Русский</Name>
+			<Synonym>
+				<v8:item>
+					<v8:lang>ru</v8:lang>
+					<v8:content>Русский</v8:content>
+				</v8:item>
+			</Synonym>
+			<Comment/>
+			<LanguageCode>ru</LanguageCode>
+		</Properties>
+	</Language>
+</MetaDataObject>