refactor(fspy-shared): make the channel allocator-generic

[wan9chi]

Motivation

The end goal is a fully std-free preload, and fspy_shared hardcoding its allocators stands in the way: channel() built its C strings with the global allocator, and sender() reached for a pooled bump internally — an infrastructure crate deciding what memory its callers should use.

channel(), Receiver, and sender() are now generic over an allocator-api2 allocator. The supervisor instantiates with Global; on the preload side the allocator is passed down from the top of the call stack — Client::from_env and Client::from_payload_bytes accept it, and the preload ctor / DllMain pass a pooled bump — so the choice of preload-safe memory lives where the attach begins, not inside library code. This also sets up the payload-view PR on top, where the allocator the client borrows becomes load-bearing for payload lifetimes.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.10%  [-12.24% .. +11.22%]  overhead  +256.20%
dynamic/access             change  +0.67%  [ -0.94% ..  +2.21%]  overhead    +6.52%
dynamic/access-relative    change  -0.35%  [ -1.62% ..  +1.32%]  overhead   +55.83%
dynamic/access-contended   change  +2.08%  [ -0.78% ..  +5.09%]  overhead    +9.77%
static/launch              change  +0.66%  [ -7.94% ..  +8.44%]  overhead  +680.17%
static/access              change  -0.19%  [ -1.49% ..  +1.79%]  overhead  +726.24%
static/access-relative     change  +0.16%  [ -1.33% ..  +1.49%]  overhead +1161.60%
static/access-contended    change  +0.36%  [ -2.00% ..  +2.22%]  overhead +3178.64%

macos

dynamic/launch             change  +0.58%  [ -5.71% ..  +6.66%]  overhead  +240.85%
dynamic/access             change  -0.34%  [-60.33% .. +16.97%]  overhead    +4.20%
dynamic/access-relative    change  +0.65%  [ -5.20% .. +11.99%]  overhead  +241.76%
dynamic/access-contended   change  +1.81%  [-15.53% .. +736.75%]  overhead    +2.31%

windows

dynamic/launch             change  -1.39%  [ -8.53% ..  +6.88%]  overhead   +25.04%
dynamic/access             change  +0.24%  [-18.05% .. +11.79%]  overhead    +1.23%
dynamic/access-relative    change  -0.34%  [ -7.52% ..  +7.65%]  overhead    +1.88%
dynamic/access-contended   change  +0.21%  [ -4.74% ..  +5.45%]  overhead    +3.45%