feat(sigsafe): resolve current directory

[wan9chi]

Motivation

Preload path resolution still obtains the current directory through nix, which may allocate through libc in interception contexts. Add caller-buffer sigsafe::fs::getcwd and its explicit-allocator adapter, then immediately use it from to_absolute_path.

The macOS open and fcntl_getpath wrappers remain private because they are implementation details of the allocation-free libSystem fast path in this slice.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.14%  [ -7.20% ..  +7.48%]  overhead   +61.20%
dynamic/access             change  -1.20%  [-16.31% .. +22.43%]  overhead   +11.08%
dynamic/access-relative    change  -9.71%  [-24.04% ..  +1.59%]  overhead   +49.24%
static/launch              change  +2.30%  [ -7.92% .. +11.06%]  overhead  +173.13%
static/access              change  +1.79%  [ -5.61% .. +15.68%]  overhead  +888.18%
static/access-relative     change  -0.24%  [ -3.31% ..  +2.43%]  overhead +1347.40%

macos

dynamic/launch             change  -0.03%  [ -1.86% ..  +1.80%]  overhead  +212.44%
dynamic/access             change  +0.00%  [ -1.76% ..  +2.05%]  overhead    +7.95%
dynamic/access-relative    change  -0.45%  [ -1.52% ..  +0.47%]  overhead  +422.89%

windows

dynamic/launch             change  +2.61%  [ -5.67% ..  +9.82%]  overhead   +26.97%
dynamic/access             change  -0.18%  [ -0.76% ..  +0.57%]  overhead    +0.94%
dynamic/access-relative    change  -0.18%  [ -2.10% ..  +1.98%]  overhead    +0.91%