refactor(fspy-client): accept allocators from the interception layer

[wan9chi]

Motivation

fspy_client_unix still created pooled bumps in the middle of call stacks for per-interception temporaries: try_handle_open drew one for path resolution, and RawExec's pointer-array build drew another during exec handling. Allocator choices belong at the top of the call stack, with everything below accepting a generic allocator — the layering the rest of the stack established.

• try_handle_open, handle_exec, and RawExec::from_exec accept the allocator for their transient path and pointer-array storage.
• The unix exec wrapper takes the allocator from each interception, so execveat — which already owns a bump for its path resolution — shares that one bump with the exec-argument rebuild instead of drawing a second chunk from the pool.

fspy_client_unix no longer creates any allocator.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -1.13%  [-13.09% .. +10.20%]  overhead  +273.63%
dynamic/access             change  -0.44%  [ -1.58% ..  +0.99%]  overhead    +6.77%
dynamic/access-relative    change  +0.91%  [ -0.45% ..  +1.75%]  overhead   +55.33%
dynamic/access-contended   change  -1.33%  [ -6.45% ..  +3.53%]  overhead    +6.94%
static/launch              change  +1.24%  [ -8.12% ..  +9.67%]  overhead  +687.37%
static/access              change  +0.58%  [ -7.75% .. +12.73%]  overhead  +748.11%
static/access-relative     change  +0.80%  [ -4.90% ..  +3.71%]  overhead +1146.30%
static/access-contended    change  -0.92%  [ -3.37% ..  +2.51%]  overhead +3263.56%

macos

dynamic/launch             change  +0.57%  [ -7.39% ..  +7.52%]  overhead  +237.84%
dynamic/access             change  +0.37%  [-10.98% .. +14.61%]  overhead    +4.09%
dynamic/access-relative    change  +0.64%  [-11.47% .. +19.85%]  overhead  +249.81%
dynamic/access-contended   change  +1.15%  [-12.26% ..  +8.91%]  overhead    +2.91%

windows

dynamic/launch             change  -1.28%  [ -8.72% ..  +6.17%]  overhead   +25.44%
dynamic/access             change  +1.49%  [ -3.76% .. +30.14%]  overhead    +2.99%
dynamic/access-relative    change  -0.88%  [-16.36% ..  +2.46%]  overhead    +2.29%
dynamic/access-contended   change  +2.34%  [ -3.52% .. +13.14%]  overhead    +2.29%