refactor(fspy-shm): generate Unix names through sigsafe

[wan9chi]

Motivation

UUID generation brings heap- and runtime-dependent machinery into Unix backing-file creation. Kernel-backed sigsafe randomness provides the same collision resistance with fixed-size buffers and is immediately used to generate the shared-memory filename.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.37%  [ -3.09% ..  +6.98%]  overhead   +53.95%
dynamic/access             change  +0.79%  [ -3.81% .. +17.01%]  overhead    +7.71%
dynamic/access-relative    change  -0.18%  [-15.31% ..  +3.65%]  overhead   +46.97%
static/launch              change  +1.38%  [ -6.53% .. +11.13%]  overhead  +153.19%
static/access              change  +0.37%  [ -7.90% .. +14.87%]  overhead  +905.04%
static/access-relative     change  -0.85%  [ -4.71% ..  +4.31%]  overhead +1391.04%

macos

dynamic/launch             change  +0.17%  [ -5.50% ..  +5.01%]  overhead  +240.75%
dynamic/access             change  +2.31%  [ -6.82% .. +134.16%]  overhead    +5.41%
dynamic/access-relative    change  -0.57%  [-16.77% .. +17.44%]  overhead  +254.66%

windows

dynamic/launch             change  +0.23%  [ -3.68% ..  +4.29%]  overhead   +26.24%
dynamic/access             change  -0.18%  [ -1.81% ..  +1.28%]  overhead    +1.31%
dynamic/access-relative    change  -0.18%  [ -1.77% ..  +0.90%]  overhead    +1.11%