refactor(fspy): extract Unix client crate

[wan9chi]

Motivation

The Unix fspy client currently lives inside the LD_PRELOAD artifact, coupling path resolution, shared-memory reporting, and exec transformation to preload-owned initialization and re-entry state.

Move the reusable client into fspy_client_unix, while leaving the constructor, global storage, and thread-local interception guard in fspy_preload_unix. This gives the injected runtime a focused client crate to adapt without inheriting preload lifecycle assumptions.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.04%  [ -6.91% ..  +6.62%]  overhead   +50.45%
dynamic/access             change  +0.10%  [ -6.77% .. +26.21%]  overhead    +7.35%
dynamic/access-relative    change  +0.77%  [-15.61% .. +22.89%]  overhead   +49.83%
static/launch              change  +1.03%  [ -8.38% ..  +9.62%]  overhead  +150.02%
static/access              change  +0.52%  [-11.47% ..  +5.61%]  overhead  +745.53%
static/access-relative     change  +1.04%  [ -8.02% ..  +8.95%]  overhead +1008.74%

macos

dynamic/launch             change  -0.02%  [ -5.01% ..  +5.05%]  overhead  +236.53%
dynamic/access             change  -2.88%  [-58.74% ..  +5.71%]  overhead    +4.04%
dynamic/access-relative    change  +1.32%  [ -8.87% .. +14.44%]  overhead  +257.66%

windows

dynamic/launch             change  +1.17%  [ -6.53% ..  +8.30%]  overhead   +26.19%
dynamic/access             change  -0.17%  [ -9.72% ..  +6.02%]  overhead    +1.21%
dynamic/access-relative    change  +1.01%  [ -9.16% .. +14.67%]  overhead    +3.94%