grammar: preserve tagged unions through singleton allOf

[joelteply]

A tagged edit argument expressed as allOf: [{"$ref": "#/definitions/EditMode"}], where EditMode is a oneOf union, currently becomes an empty-object-only grammar. The union is ignored by the allOf property collector, so a valid search_replace argument is rejected while {} is admitted.

Delegate a transparent singleton union wrapper to the existing union visitor. Only annotation/definition siblings are allowed; semantic siblings and general allOf intersections retain their existing behavior. The Qwen3.5 fixture uses the captured full edit schema and exercises actual grammar acceptance/rejection plus XML argument round-tripping.

Validation on Windows MSVC Release CPU-only:

• Same always-active fixture with the old converter rejects valid search_replace (Expected 1, Actual 0).
• Restored fix passes the Qwen3.5 template tests, including append/search_replace, empty/unknown rejection, and sibling constraints.
• Existing test-json-schema-to-grammar passes.

This PR is stacked on dc3a1f4c/container-reader-custody at 3794294, the current reviewed native pin. It does not promote an engine or change serving settings. The native bug also exists in the older 178 baseline; newer wrapped tool schemas expose it. This does not establish that every observed coding stall has the same cause.

AI assistance: Codex implemented and reviewed the narrow fix under the maintainer's explicit direction; independent agent source review and compiled before/after evidence are recorded in the governed task25c6a4dd.

Fresh ARM64 CI found two existing speculative batch aggregates missing the final label_weight initializer under -Werror. Both now explicitly initialize nullptr, preserving their prior zero initialization without warning suppression. Focused Windows compilation and Qwen regression pass after this correction; fresh hosted checks remain required.

Further fresh CI exposed preexisting test-walk-exact build defects: unsigned window bound compared to signed loop index, and private symbols linked from a Windows DLL. The loop now uses uint32_t; the unchanged target is registered beside existing internal tests under NOT WIN32 OR NOT BUILD_SHARED_LIBS, retaining Linux and Windows static coverage. Windows static test-walk-exact compiles and links; no assertion or model scenario was removed.