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/runner.mjs

@@ -223,8 +223,16 @@ function buildArgs(skillConfig, caseData, workDir, inputFilePath, runtime) {
       case 'workPath':
         // workDir + value from case.params or case (specified in mapping.field)
         const wpField = mapping.field || 'objectPath';
-        const wpVal = caseData.params?.[wpField] ?? caseData[wpField] ?? '';
-        args.push(join(workDir, wpVal));
+        const wpVal = caseData.params?.[wpField] ?? caseData[wpField];
+        if (wpVal === undefined || wpVal === null || wpVal === '') {
+          if (mapping.optional) {
+            args.pop(); // remove the flag we pushed at the top of the loop
+            break;
+          }
+          args.push(join(workDir, ''));
+        } else {
+          args.push(join(workDir, wpVal));
+        }
         break;
       case 'switch':
         // flag already pushed, no value needed — remove the flag and re-push conditionally