feat(fspy-nostd): add Unix owned-file access

[wan9chi]

Motivation

The injected runtime must open existing Unix backing files without Rust std or libc on Linux. This adds openat, fstat, and owned-descriptor access to fspy_nostd and immediately adopts them in fspy_shm::open, while preserving the existing OsStr public API.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.55%  [ -9.91% .. +11.15%]  overhead   +55.00%
dynamic/access             change  +1.74%  [ -9.90% .. +19.37%]  overhead    +3.56%
dynamic/access-relative    change  -0.52%  [-21.02% .. +20.98%]  overhead   +42.48%
static/launch              change  +2.57%  [ -7.61% .. +13.39%]  overhead  +150.38%
static/access              change  -2.35%  [-18.08% .. +10.26%]  overhead  +881.54%
static/access-relative     change  -0.12%  [ -7.09% ..  +6.60%]  overhead +1200.05%

macos

dynamic/launch             change  +0.22%  [ -3.07% ..  +3.57%]  overhead  +214.42%
dynamic/access             change  +1.51%  [ -6.86% .. +135.53%]  overhead    +7.19%
dynamic/access-relative    change  -0.33%  [ -7.36% ..  +7.72%]  overhead  +210.16%

windows

dynamic/launch             change  +0.50%  [ -5.34% ..  +6.00%]  overhead   +25.24%
dynamic/access             change  -0.17%  [ -4.91% ..  +8.33%]  overhead    +1.66%
dynamic/access-relative    change  -0.17%  [ -6.67% ..  +3.02%]  overhead    +1.64%

[wan9chi]

Superseded by #664, which combines the Unix adoption into one PR.