refactor(fspy-shm): split platform implementations

[wan9chi]

Motivation

fspy_shm needs separate Unix and Windows implementation seams before their low-level dependencies diverge. This moves the existing behavior into platform modules and centralizes the cross-platform behavioral tests without changing the public API or runtime behavior.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -1.20%  [-13.03% .. +13.91%]  overhead   +55.65%
dynamic/access             change  +0.59%  [-11.91% .. +18.51%]  overhead    +4.74%
dynamic/access-relative    change  +0.22%  [-15.96% .. +21.01%]  overhead   +48.67%
static/launch              change  +0.78%  [-12.66% .. +14.04%]  overhead  +159.66%
static/access              change  -0.78%  [-11.46% ..  +8.04%]  overhead  +782.36%
static/access-relative     change  -0.82%  [ -7.39% ..  +6.13%]  overhead +1244.01%

macos

dynamic/launch             change  -0.39%  [ -3.11% ..  +3.40%]  overhead  +215.85%
dynamic/access             change  -0.72%  [ -5.13% ..  +2.93%]  overhead    +4.76%
dynamic/access-relative    change  +0.62%  [ -1.04% ..  +1.59%]  overhead  +266.43%

windows

dynamic/launch             change  -1.21%  [ -6.25% ..  +4.60%]  overhead   +24.41%
dynamic/access             change  +0.18%  [-11.26% ..  +7.64%]  overhead    +1.42%
dynamic/access-relative    change  +0.35%  [ -2.69% ..  +6.90%]  overhead    +1.62%