refactor(fspy-shm): open backing files through sigsafe

[wan9chi]

Motivation

The injected runtime may attach before libc initialization, while fspy_shm::open used std::fs::OpenOptions. This PR routes Linux backing-file opens through the sigsafe raw openat wrapper with a caller-owned path buffer, preserving the existing fspy_shm API and storage behavior.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.33%  [ -7.44% ..  +8.59%]  overhead   +61.14%
dynamic/access             change  +0.39%  [-17.42% .. +13.88%]  overhead    +7.94%
dynamic/access-relative    change  +0.29%  [-21.57% .. +21.49%]  overhead   +43.14%
static/launch              change  +0.98%  [ -7.49% ..  +9.59%]  overhead  +181.74%
static/access              change  -0.74%  [ -7.39% ..  +3.56%]  overhead  +874.06%
static/access-relative     change  +0.18%  [ -4.62% ..  +6.40%]  overhead +1335.18%

macos

dynamic/launch             change  -0.39%  [ -5.67% ..  +5.60%]  overhead  +241.05%
dynamic/access             change  -0.86%  [-10.69% ..  +8.36%]  overhead    +4.32%
dynamic/access-relative    change  +3.32%  [ -9.34% .. +59.79%]  overhead  +246.20%

windows

dynamic/launch             change  +0.44%  [ -4.83% ..  +5.71%]  overhead   +28.77%
dynamic/access             change  -0.54%  [ -5.88% ..  +2.34%]  overhead    +1.79%
dynamic/access-relative    change  -0.17%  [ -1.91% ..  +1.25%]  overhead    +1.60%