refactor(fspy-shm): adopt fspy_nostd on Unix

[wan9chi]

Motivation

The injected runtime needs the Unix kernel-facing shared-memory operations to be reusable without Rust std or libc on Linux. This PR routes open, stat, mapping, creation, resizing, and removal through fspy_nostd as one vertical change. The existing fspy_shm public API and Windows implementation remain unchanged, and every added primitive is immediately consumed.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.25%  [ -2.81% ..  +4.49%]  overhead   +48.21%
dynamic/access             change  +0.28%  [ -0.70% ..  +1.35%]  overhead    +6.64%
dynamic/access-relative    change  -0.19%  [ -0.92% ..  +0.51%]  overhead   +46.85%
static/launch              change  +0.18%  [ -3.76% ..  +4.64%]  overhead  +143.50%
static/access              change  -0.44%  [ -1.29% ..  +0.67%]  overhead  +812.31%
static/access-relative     change  -0.24%  [ -0.94% ..  +0.68%]  overhead +1319.28%

macos

dynamic/launch             change  -0.11%  [ -4.06% ..  +4.76%]  overhead  +218.87%
dynamic/access             change  -1.68%  [ -5.88% ..  +3.62%]  overhead    +3.62%
dynamic/access-relative    change  +5.64%  [ -8.43% .. +18.55%]  overhead  +260.52%

windows

dynamic/launch             change  -0.24%  [ -7.57% ..  +6.42%]  overhead   +23.98%
dynamic/access             change  +0.18%  [ -9.40% .. +20.80%]  overhead    +1.63%
dynamic/access-relative    change  -0.17%  [ -7.61% ..  +5.56%]  overhead    +2.18%