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

[wan9chi]

Motivation

After opening through sigsafe, Unix 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 returns to std for metadata or descriptor teardown.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.58%  [ -6.86% ..  +5.91%]  overhead   +65.39%
dynamic/access             change  +0.21%  [ -2.90% ..  +2.80%]  overhead    +5.67%
dynamic/access-relative    change  -0.31%  [ -2.91% ..  +0.75%]  overhead   +52.03%
static/launch              change  +2.30%  [ -8.33% .. +10.98%]  overhead  +194.08%
static/access              change  +1.24%  [-10.54% ..  +9.60%]  overhead  +739.99%
static/access-relative     change  -0.35%  [ -3.92% ..  +3.37%]  overhead +1154.16%

macos

dynamic/launch             change  -0.62%  [ -5.35% ..  +4.17%]  overhead  +221.35%
dynamic/access             change  +0.60%  [ -8.06% .. +43.51%]  overhead    +3.45%
dynamic/access-relative    change  +6.18%  [ -7.72% .. +80.94%]  overhead  +262.38%

windows

dynamic/launch             change  -2.01%  [-12.29% ..  +9.92%]  overhead   +21.90%
dynamic/access             change  +2.09%  [-12.67% .. +20.16%]  overhead    +1.25%
dynamic/access-relative    change  +0.86%  [-13.00% .. +24.74%]  overhead    +2.35%