Isolate V8 toolchain discovery from inherited Visual Studio state #168

Problem

The Windows native runtime package job prepares Dawn and ANGLE before invoking scripts/build-native-engine-runtime.ps1. Those actions intentionally leave the outer process in an initialized x64 Visual Studio environment. The V8/depot_tools bootstrap inherits that state, then Chromium's GN setup requests its own x86 environment:

vcvarsall.bat amd64_x86 10.0.26100.0
... failed with error 255

Exact stacked evidence: NuGet packages run 35123381474, attempt 2, job 104896221550. The preceding Dawn and ANGLE preparation succeeded; V8 GN failed in build/toolchain/win/setup_toolchain.py.

Scope

• Give only the V8/depot_tools subprocess chain a clean Windows child environment.
• Restore PATH from __VSCMD_PREINIT_PATH and remove inherited Visual Studio/compiler/SDK variables before V8 checkout, GN generation, and Ninja.
• Preserve unrelated runner, package, and WebScene variables.
• Leave the caller/outer environment and the later native WebScene x64 build environment unchanged.
• Fail closed if an initialized developer environment does not expose its original path.
• Add meaningful regression tests for child isolation, caller preservation, unrelated-variable preservation, and fail-closed behavior.
• Re-run the cumulative three-RID package/evidence/package-inventory/consumer matrix.

Acceptance

• Regression tests pass locally and in CI.
• Exact Windows package job passes Dawn → ANGLE → V8 → native runtime build/tests.
• Required and candidate evidence include osx-arm64, linux-x64, and win-x64.
• Release package inventory and installed-package consumer tests pass for all three RIDs.

Blocks #161 and native Stack #157. Related to #79, #80, #165, #166, and draft consolidation PR #76.