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

[wan9chi]

Motivation

The injected runtime may attach before libc initialization on Linux, while Unix fspy_shm::open used std::fs::OpenOptions. This PR routes Unix backing-file opens through the sigsafe openat wrapper with a caller-owned path buffer. Linux uses the raw-syscall implementation; macOS uses sigsafe’s libSystem implementation. The existing fspy_shm API and storage behavior remain unchanged.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.34%  [ -5.40% ..  +6.36%]  overhead   +51.81%
dynamic/access             change  -0.41%  [-20.01% ..  +5.52%]  overhead    +3.32%
dynamic/access-relative    change  -0.33%  [-13.98% ..  +4.17%]  overhead   +53.88%
static/launch              change  +1.27%  [ -5.66% ..  +8.75%]  overhead  +146.29%
static/access              change  +0.22%  [ -7.59% ..  +8.61%]  overhead  +702.51%
static/access-relative     change  -0.92%  [ -7.14% ..  +2.68%]  overhead +1133.10%

macos

dynamic/launch             change  +0.30%  [ -2.36% ..  +2.92%]  overhead  +239.91%
dynamic/access             change  +0.00%  [ -2.74% ..  +3.19%]  overhead    +1.42%
dynamic/access-relative    change  +1.09%  [ -3.75% ..  +3.75%]  overhead  +254.81%

windows

dynamic/launch             change  +0.13%  [ -2.68% ..  +5.17%]  overhead   +25.63%
dynamic/access             change  +0.00%  [ -0.76% ..  +1.14%]  overhead    +1.55%
dynamic/access-relative    change  +0.00%  [ -0.73% ..  +0.73%]  overhead    +1.29%