refactor(sigsafe): extract allocator crate

[wan9chi]

Motivation

Keep the low-level syscall wrapper crate independent from allocator policy and dependencies, while preserving the malloc-free arena used by preload code.

Moves the existing arena, mmap allocator, and lock-free chunk pool into a new sigsafe_alloc crate. The preload library now depends on that crate for arena(), while sigsafe_alloc builds on sigsafe for mmap and page-size primitives.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.62%  [ -2.57% ..  +2.78%]  overhead   +54.49%
dynamic/access             change  +0.14%  [ -0.69% ..  +1.59%]  overhead    +6.68%
dynamic/access-relative    change  +0.09%  [ -0.95% ..  +0.98%]  overhead   +54.36%
static/launch              change  +0.97%  [ -2.80% ..  +4.83%]  overhead  +161.30%
static/access              change  +0.05%  [ -0.83% ..  +1.12%]  overhead  +816.56%
static/access-relative     change  +0.34%  [ -1.04% ..  +1.04%]  overhead +1321.41%

macos

dynamic/launch             change  -0.08%  [ -2.72% ..  +3.05%]  overhead  +212.33%
dynamic/access             change  -0.35%  [ -3.23% ..  +2.42%]  overhead    +1.70%
dynamic/access-relative    change  +0.08%  [ -1.57% ..  +1.38%]  overhead  +379.12%

windows

dynamic/launch             change  -0.12%  [ -1.84% ..  +1.76%]  overhead   +27.86%
dynamic/access             change  +0.00%  [ -0.57% ..  +0.75%]  overhead    +1.54%
dynamic/access-relative    change  +0.00%  [ -0.55% ..  +0.75%]  overhead    +1.32%