feat(fspy-nostd): add Unix file lifecycle operations

[wan9chi]

Motivation

Unix shared-memory creation, resizing, and keeper cleanup still depended on std::fs. This routes those operations through fspy_nostd using openat, ftruncate, and unlinkat, completing the low-level Unix migration without changing the public fspy_shm API.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.68%  [ -7.10% .. +10.66%]  overhead   +54.56%
dynamic/access             change  +0.12%  [-21.97% ..  +9.79%]  overhead    +6.83%
dynamic/access-relative    change  +0.09%  [-14.78% ..  +8.31%]  overhead   +47.48%
static/launch              change  +0.52%  [ -7.76% ..  +8.17%]  overhead  +144.36%
static/access              change  -1.01%  [ -9.44% ..  +4.09%]  overhead  +852.77%
static/access-relative     change  -0.68%  [ -5.42% ..  +4.59%]  overhead +1364.71%

macos

dynamic/launch             change  +1.23%  [ -3.25% ..  +6.23%]  overhead  +225.94%
dynamic/access             change  +0.33%  [ -3.24% ..  +3.80%]  overhead    +3.91%
dynamic/access-relative    change  +0.32%  [ -5.27% ..  +5.38%]  overhead  +245.04%

windows

dynamic/launch             change  +0.37%  [ -5.92% ..  +6.83%]  overhead   +27.53%
dynamic/access             change  +0.00%  [ -1.10% ..  +1.47%]  overhead    +1.31%
dynamic/access-relative    change  +0.53%  [ -1.08% ..  +2.15%]  overhead    +1.66%

[wan9chi]

Superseded by #664, which combines the Unix adoption into one PR.