refactor(fspy): retain C string lengths in path resolution

[wan9chi]

Motivation

Path resolution currently converts sigsafe::CStr<Thin> back into core::ffi::CStr, so its terminator scan produces only a temporary slice. Count into sigsafe::CStr<Fat>, expose its bytes without the terminator, and use that retained length while resolving paths.

This keeps the callback and allocation behavior unchanged so the following PR can review the output-ownership change independently.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.43%  [ -5.90% ..  +5.39%]  overhead   +62.59%
dynamic/access             change  +0.57%  [ -8.81% .. +11.59%]  overhead    +8.08%
dynamic/access-relative    change  +0.58%  [-10.25% .. +19.69%]  overhead   +49.79%
static/launch              change  +1.17%  [ -6.25% ..  +8.68%]  overhead  +180.65%
static/access              change  +0.10%  [ -7.41% .. +11.64%]  overhead  +826.57%
static/access-relative     change  -1.41%  [ -8.49% ..  +7.77%]  overhead +1215.93%

macos

dynamic/launch             change  -0.14%  [ -3.30% ..  +3.49%]  overhead  +220.11%
dynamic/access             change  +0.73%  [ -1.08% ..  +3.58%]  overhead    +4.04%
dynamic/access-relative    change  +0.11%  [ -5.22% ..  +5.34%]  overhead  +250.89%

windows

dynamic/launch             change  -0.16%  [ -7.67% ..  +4.71%]  overhead   +26.64%
dynamic/access             change  -0.18%  [ -1.61% ..  +1.45%]  overhead    +1.87%
dynamic/access-relative    change  +0.54%  [ -0.36% ..  +1.65%]  overhead    +1.70%