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

[wan9chi]

Motivation

After opening through raw syscalls, Linux fspy_shm still queried metadata and owned the descriptor through std::fs::File. This PR reads the backing size with sigsafe fstat and retains a sigsafe OwnedFd, so attachment no longer depends on libc for metadata or descriptor teardown.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.81%  [ -5.66% ..  +7.53%]  overhead   +53.05%
dynamic/access             change  +0.60%  [ -3.88% ..  +4.74%]  overhead    +5.46%
dynamic/access-relative    change  -0.63%  [-11.13% .. +25.87%]  overhead   +45.25%
static/launch              change  +1.06%  [ -8.02% .. +11.10%]  overhead  +151.54%
static/access              change  -0.19%  [ -6.75% ..  +8.06%]  overhead  +775.57%
static/access-relative     change  -0.08%  [ -6.98% ..  +5.85%]  overhead +1115.93%

macos

dynamic/launch             change  -0.39%  [ -5.04% ..  +5.52%]  overhead  +246.06%
dynamic/access             change  +1.03%  [-47.75% .. +25.31%]  overhead    +1.77%
dynamic/access-relative    change  +0.66%  [-29.93% .. +33.04%]  overhead  +236.54%

windows

dynamic/launch             change  +1.42%  [ -6.48% ..  +7.18%]  overhead   +27.51%
dynamic/access             change  +0.00%  [ -0.91% ..  +1.94%]  overhead    +1.66%
dynamic/access-relative    change  +0.35%  [ -1.60% ..  +1.95%]  overhead    +1.62%