refactor(fspy): rename sigsafe crates

[wan9chi]

Motivation

sigsafe described only the Unix signal-handler constraint, but the crate is becoming the shared no_std layer for Unix preload code, Windows loader callbacks, process startup, and Linux runtime injection. Rename it to fspy_nostd and rename the paired allocator crate to fspy_nostd_alloc so their names match that broader role.

This updates workspace dependencies and documentation without changing runtime behavior.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.61%  [ -3.67% ..  +5.39%]  overhead   +47.60%
dynamic/access             change  -0.40%  [ -1.09% ..  +0.66%]  overhead    +5.57%
dynamic/access-relative    change  -0.40%  [ -1.40% ..  +0.30%]  overhead   +45.31%
static/launch              change  +0.93%  [ -3.98% ..  +6.13%]  overhead  +140.24%
static/access              change  -0.17%  [ -1.43% ..  +1.35%]  overhead  +812.96%
static/access-relative     change  +0.03%  [ -1.00% ..  +0.97%]  overhead +1320.18%

macos

dynamic/launch             change  -0.67%  [ -3.87% ..  +2.64%]  overhead  +224.44%
dynamic/access             change  +0.00%  [ -3.76% ..  +3.17%]  overhead    +2.49%
dynamic/access-relative    change  +0.75%  [ -2.04% ..  +2.95%]  overhead  +239.32%

windows

dynamic/launch             change  -0.05%  [ -5.08% ..  +6.26%]  overhead   +28.01%
dynamic/access             change  +0.00%  [ -1.08% ..  +0.93%]  overhead    +2.01%
dynamic/access-relative    change  +0.18%  [ -0.72% ..  +1.24%]  overhead    +1.81%