Merge static_h branch from upstream Hermes repo

[vmoroz]

Summary

Integrates the upstream static_h branch from facebook/hermes
(commit 657380912e) into hermes-windows. The static_h branch is the
new default development branch in upstream Hermes, replacing main.

This PR also introduces a pluggable Intl (ECMA-402) implementation for
Windows, adopts the @rnx-kit/fork-sync tool for future upstream syncs,
and fixes previously suppressed JS test failures.

---

1. Upstream static_h Integration

The static_h branch brings a major architectural change to Hermes:
a new static compilation pipeline and accompanying runtime support.
Merging it required resolving conflicts across ~6,000 files and adapting
the Windows build for new upstream patterns.

Key integration fixes:

• Moved external/boost to external/boost2 to avoid conflict with
  the upstream-added Boost dependency.
• Fixed compilation issues for x64, x86, ARM64, and ARM64EC with Clang
  and MSVC (lld-link specifics, visibility, stack alignment).
• Fixed x86 stack overflow in shermes (stack frame size difference
  between 32-bit and 64-bit).
• Fixed ARM64EC build compatibility.
• Fixed UWP compilation.

2. Fork Sync Tooling & Developer Script

Adopted @rnx-kit/fork-sync to manage upstream synchronization. Config
files added:

• sync-config.json — tracks the upstream repo, branch, and last-synced
  commit.
• sync-manifest.json — declares sync dependencies (hermes core,
  icu-small, test262).
• tools/fork-sync/package.json — local fork-sync package.

Added dev.ps1 — a PowerShell task runner as a single entry point for
common developer tasks:

• .\dev build [args] — builds Hermes (wraps build.js).
• .\dev fork-sync --dep <name> — runs fork-sync for a dependency.
• Auto-installs fork-sync npm dependencies on first use.

3. Pluggable Intl Implementation (Experimental)

Added a new ECMA-402 (Intl) implementation for Windows with a
runtime-selectable ICU provider architecture. All Windows-specific code
lives in lib/Platform/Intl/impl_win/, minimizing upstream merge
friction.

Architecture: A pure-C ICU vtable (hermes_icu.h) with ~260
function pointers matching ICU C API signatures. No static ICU linking
— all ICU access is via LoadLibrary/GetProcAddress at runtime.

Providers (resolved at runtime):

1. Bundled ICU — hermes-icu.dll built from vendored ICU 78
   (sourced from Node.js icu-small). Full ECMA-402 compliance.
2. System ICU — Windows 10 1903+ built-in icu.dll. Full ECMA-402
   for most features.
3. Dynamic ICU — Host-supplied ICU DLLs from any path.
4. WinGlob — Windows NLS API fallback, always available. Basic Intl
   support without ICU.
5. Host vtable — Custom function pointer table provided by the host.

Test results:

• Test262 intl402: 348 pass / 0 fail (of 348 executed; 1167 skipped for
  features Hermes does not implement, e.g. Locale, PluralRules,
  Segmenter).
• Hermes JS lit tests: 2888 pass / 0 fail (x64), 2887 pass / 0 fail
  (x86).

New test infrastructure:

• tools/hermes_rt/ — lightweight JS runner using the public C API,
  supports --intl-provider flag for testing each provider.
• external/test262/ — pinned Test262 suite for intl402 conformance.
• build.js --test262-intl — new CI integration for running intl tests.

Note on ICU landscape (from investigation): The ICU C API has
significant gaps vs. the C++ API, making full ECMA-402 compliance
through C-only wrappers challenging. The bundled hermes-icu.dll
(which uses C++ internally) is the recommended path. System ICU and
WinGlob providers have inherent limitations.

4. JS Test Fixes

Removed lit.local.cfg test suppressions introduced in commit
725f1bd8 (issue #230) by fixing the underlying test issues:

• TypedArray.js, array-functions.js, number-functions.js,
  proxy.js, string-functions.js — all fixed and re-enabled.
• test/BCGen/HBC/deltamode/string-functions-update.js — re-enabled.
• test/hermes/intl/ — re-enabled (now backed by the new Intl
  implementation).

5. Vendored Dependencies

• external/icu-small/ — ICU 78 source from Node.js, builds
  hermes-icu.dll (~35 MB). Single export: hermes_icu_get_vtable.
• external/test262/ — Pinned ECMA-402 test suite.

Build & Test

All platforms compile. JS tests pass on x64 and x86. ARM64 and ARM64EC
compile but tests are not run (cross-compilation).

.\dev build --platform x64              # build
.\dev build --jstest                    # JS lit tests
.\dev build --test262-intl              # ECMA-402 tests
.\dev fork-sync --dep icu-small         # sync icu-small from upstream