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 inside the existing to_absolute_path callback contract.

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

That fast path stops short of libSystem's verification step. libSystem stats the path F_GETPATH returned and compares it against the descriptor, falling back to walking up through .. when they disagree. This wrapper has no slow path to fall back to, so the comparison could only turn a slightly stale answer into a hard error — and its caller records paths a process accessed rather than resolving them, where a stale name is a cosmetic inaccuracy but an error is a lost access. Skipping it also keeps the most common resolution to two syscalls.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.12%  [ -8.76% ..  +8.29%]  overhead   +53.55%
dynamic/access             change  -0.25%  [-14.96% .. +12.61%]  overhead    +6.31%
dynamic/access-relative    change  -2.66%  [-13.50% .. +10.71%]  overhead   +52.30%
static/launch              change  -0.43%  [ -8.67% ..  +7.64%]  overhead  +148.49%
static/access              change  +0.25%  [-12.29% .. +12.05%]  overhead  +820.51%
static/access-relative     change  +0.50%  [ -3.74% ..  +5.37%]  overhead +1312.56%

macos

dynamic/launch             change  +0.46%  [ -4.67% ..  +4.80%]  overhead  +223.66%
dynamic/access             change  +0.34%  [ -1.77% ..  +2.33%]  overhead    +2.87%
dynamic/access-relative    change -30.16%  [-31.42% .. -28.13%]  overhead  +231.11%

windows

dynamic/launch             change  -0.66%  [ -4.59% ..  +3.26%]  overhead   +26.81%
dynamic/access             change  +0.18%  [ -3.33% ..  +2.03%]  overhead    +1.27%
dynamic/access-relative    change  +0.00%  [ -2.45% ..  +2.14%]  overhead    +1.63%