refactor(fspy-shm): make Unix implementation no-std

[wan9chi]

Motivation

The injected runtime cannot rely on Rust std or libc initialization. Unix fspy_shm already performs every operation through sigsafe, so exposing std::io::Error and retaining std implementation types is unnecessary. Use sigsafe Error and Result aliases, core types, and a crate-level Unix no_std guard; Windows keeps its std::io API.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.33%  [ -6.10% ..  +7.86%]  overhead   +50.86%
dynamic/access             change  -0.53%  [ -5.80% ..  +1.13%]  overhead    +7.14%
dynamic/access-relative    change  -0.58%  [ -9.51% ..  +1.03%]  overhead   +46.73%
static/launch              change  +1.62%  [ -9.12% .. +12.41%]  overhead  +142.76%
static/access              change  +0.18%  [-10.40% ..  +9.50%]  overhead  +825.90%
static/access-relative     change  -0.46%  [ -2.30% ..  +2.70%]  overhead +1331.17%

macos

dynamic/launch             change  +0.25%  [ -2.72% ..  +3.09%]  overhead  +220.79%
dynamic/access             change  +1.10%  [ -3.90% ..  +5.09%]  overhead    +5.54%
dynamic/access-relative    change  +0.00%  [ -2.92% ..  +3.44%]  overhead  +251.20%

windows

dynamic/launch             change  +2.57%  [ -5.34% .. +10.11%]  overhead   +27.03%
dynamic/access             change  -0.53%  [ -2.64% ..  +1.81%]  overhead    +1.68%
dynamic/access-relative    change  -0.85%  [-18.10% ..  +6.71%]  overhead    +1.75%