feat(fspy-nostd): add Unix file mappings

[wan9chi]

Motivation

Unix shared-memory mappings still went through memmap2, which is not suitable for the injected runtime. This exposes file-backed mapping through fspy_nostd and immediately adopts it in fspy_shm; Windows continues using memmap2 unchanged.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.11%  [ -6.70% ..  +9.72%]  overhead   +54.73%
dynamic/access             change  +1.41%  [ -5.47% .. +11.45%]  overhead    +6.33%
dynamic/access-relative    change  +0.05%  [ -9.86% ..  +9.09%]  overhead   +44.07%
static/launch              change  +1.45%  [ -7.22% .. +10.97%]  overhead  +158.09%
static/access              change  +0.31%  [ -5.15% ..  +5.27%]  overhead  +717.84%
static/access-relative     change  +0.33%  [ -5.67% ..  +7.56%]  overhead +1027.80%

macos

dynamic/launch             change  -0.49%  [ -5.03% ..  +4.71%]  overhead  +247.97%
dynamic/access             change  +0.00%  [ -5.12% ..  +5.84%]  overhead    +6.80%
dynamic/access-relative    change  -1.35%  [-16.58% .. +21.40%]  overhead  +240.95%

windows

dynamic/launch             change  -0.92%  [-10.98% .. +11.10%]  overhead   +27.23%
dynamic/access             change  +0.18%  [ -6.58% ..  +5.31%]  overhead    +1.64%
dynamic/access-relative    change  -0.34%  [-16.37% .. +25.37%]  overhead    +1.95%

[wan9chi]

Superseded by #664, which combines the Unix adoption into one PR.