feat(sigsafe): resolve Linux descriptor paths

[wan9chi]

Motivation

Linux descriptor path resolution still uses nix::readlink and heap-backed formatting. Add a raw caller-buffer readlinkat wrapper and allocator adapter, then immediately replace that path with stack formatting and arena storage.

The low-level wrapper preserves readlink truncation semantics; the allocating adapter retries only when the buffer is filled.

[github-actions]

fspy benchmark

linux

macos

dynamic/launch             change  +0.04%  [ -4.41% ..  +5.03%]  overhead  +230.24%
dynamic/access             change  +1.32%  [ -7.65% .. +137.22%]  overhead    +6.55%
dynamic/access-relative    change  -0.31%  [ -6.02% ..  +3.01%]  overhead  +379.59%

windows

dynamic/launch             change  -0.03%  [ -2.58% ..  +1.50%]  overhead   +27.91%
dynamic/access             change  +0.00%  [ -0.76% ..  +1.17%]  overhead    +1.76%
dynamic/access-relative    change  +0.19%  [ -0.57% ..  +0.96%]  overhead    +1.32%