refactor(fspy-shm): adopt fspy_nostd on Windows

[wan9chi]

Motivation

The Windows preload opens and maps shared memory before main and under the loader lock, where Rust std lazy initialization is not a dependable foundation. This moves the Windows implementation onto no-std Win32 calls while preserving the existing fspy_shm API.

It is the Windows counterpart to #664: fspy_nostd provides small safe wrappers where it can enforce path, handle, or mapping ownership. fspy_shm calls DeviceIoControl and SetFileInformationByHandle directly because their buffer contracts and flags are specific to its sparse-file, sizing, and removal policy. The outer OsStr and io::Error conversion remains for the later public API migration.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.89%  [ -9.51% .. +17.83%]  overhead   +51.62%
dynamic/access             change  -0.49%  [-24.24% .. +23.67%]  overhead    +6.33%
dynamic/access-relative    change  -0.82%  [-24.55% .. +15.00%]  overhead   +43.70%
static/launch              change  +1.80%  [ -9.08% .. +13.61%]  overhead  +151.73%
static/access              change  +1.59%  [ -4.75% .. +17.66%]  overhead  +958.66%
static/access-relative     change  -1.81%  [ -6.77% ..  +3.85%]  overhead +1349.55%

macos

dynamic/launch             change  +0.09%  [ -1.92% ..  +2.00%]  overhead  +217.30%
dynamic/access             change  -0.37%  [ -3.71% ..  +2.58%]  overhead    +3.26%
dynamic/access-relative    change  -0.10%  [ -2.38% ..  +1.66%]  overhead  +265.91%

windows

dynamic/launch             change  +0.63%  [ -6.41% .. +11.27%]  overhead   +28.09%
dynamic/access             change  -0.26%  [ -5.25% ..  +3.20%]  overhead    +2.21%
dynamic/access-relative    change  -0.76%  [ -4.99% ..  +2.48%]  overhead    +2.29%