refactor(fspy-shm): map backing files through sigsafe

[wan9chi]

Motivation

Linux fspy_shm still mapped and unmapped through memmap2. This PR backs Mapping with sigsafe file-backed mmap and munmap plus a non-null pointer and length on Linux, completing a libc-independent open, map, and drop path usable before libc initialization while retaining memmap2 elsewhere.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.82%  [ -3.64% ..  +6.49%]  overhead   +49.09%
dynamic/access             change  -0.39%  [ -1.48% ..  +0.91%]  overhead    +6.46%
dynamic/access-relative    change  +1.81%  [ -0.09% ..  +3.08%]  overhead   +47.61%
static/launch              change  +1.23%  [ -3.82% ..  +6.45%]  overhead  +151.07%
static/access              change  -0.23%  [ -2.46% ..  +1.72%]  overhead  +825.51%
static/access-relative     change  +0.17%  [ -1.38% ..  +1.53%]  overhead +1336.58%

macos

dynamic/launch             change  +0.39%  [ -2.78% ..  +2.50%]  overhead  +218.43%
dynamic/access             change  +0.83%  [ -4.12% ..  +7.06%]  overhead    +7.81%
dynamic/access-relative    change  -0.33%  [ -4.81% ..  +2.05%]  overhead  +254.79%

windows

dynamic/launch             change  +0.02%  [ -4.08% ..  +3.46%]  overhead   +26.16%
dynamic/access             change  +0.00%  [ -6.14% ..  +2.49%]  overhead    +1.41%
dynamic/access-relative    change  +0.21%  [ -3.35% .. +20.57%]  overhead    +1.40%