refactor(fspy): type intercepted paths

[wan9chi]

Motivation

Preload path arguments enter as raw ABI values, while the later signal-safe filesystem work needs borrowed, typed inputs. Introduce the minimal sigsafe::CStr<Thin> representation, expose rustix's borrowed descriptor types, and convert raw descriptors and path pointers at the interception boundary. This makes ToAbsolutePath safe without introducing length metadata, allocation, or filesystem APIs early.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.18%  [ -3.66% ..  +7.02%]  overhead   +53.78%
dynamic/access             change  -0.26%  [-12.18% .. +15.18%]  overhead    +7.01%
dynamic/access-relative    change  +0.00%  [-11.39% ..  +2.82%]  overhead   +54.81%
static/launch              change  +1.41%  [ -6.33% .. +11.84%]  overhead  +150.96%
static/access              change  +0.14%  [ -3.38% ..  +6.77%]  overhead  +861.28%
static/access-relative     change  -0.74%  [ -5.06% ..  +2.70%]  overhead +1168.49%

macos

dynamic/launch             change  -0.44%  [ -5.41% ..  +5.74%]  overhead  +237.87%
dynamic/access             change  +0.32%  [ -6.76% ..  +4.17%]  overhead    +1.69%
dynamic/access-relative    change  -0.45%  [ -6.07% ..  +6.35%]  overhead  +372.92%

windows

dynamic/launch             change  +0.29%  [ -3.31% ..  +3.34%]  overhead   +28.71%
dynamic/access             change  +0.00%  [ -1.08% ..  +1.12%]  overhead    +1.15%
dynamic/access-relative    change  +0.19%  [ -0.90% ..  +1.14%]  overhead    +1.52%