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; the preloads pass sender() a pooled bump at the call site, keeping the choice of preload-safe memory where the constraint lives. This also sets up the payload-view PR on top, where the preload's allocator choice becomes load-bearing for payload lifetimes.

The lock file path is also generated absolute now: temp_dir() reflects TMPDIR verbatim, which may be relative, and the path travels to processes with other working directories — fixing a latent relative-TMPDIR hazard the shm backing path already handled.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.02%  [ -3.18% ..  +6.51%]  overhead   +52.56%
dynamic/access             change  +0.40%  [ -0.67% ..  +1.25%]  overhead    +5.71%
dynamic/access-relative    change  +0.09%  [ -1.02% ..  +1.02%]  overhead   +46.28%
static/launch              change  +0.29%  [ -4.49% ..  +7.15%]  overhead  +143.07%
static/access              change  +0.05%  [ -1.11% ..  +1.33%]  overhead  +809.29%
static/access-relative     change  +0.30%  [ -1.66% ..  +2.36%]  overhead +1318.55%

macos

dynamic/launch             change  -0.16%  [ -4.83% ..  +4.39%]  overhead  +238.06%
dynamic/access             change  +1.07%  [ -6.94% .. +27.51%]  overhead    +6.42%
dynamic/access-relative    change  +0.46%  [ -7.90% ..  +7.94%]  overhead  +215.38%

windows

dynamic/launch             change  -0.00%  [ -4.23% ..  +4.86%]  overhead   +26.66%
dynamic/access             change  -0.54%  [ -2.39% ..  +1.09%]  overhead    +1.31%
dynamic/access-relative    change  +0.18%  [ -1.61% ..  +1.13%]  overhead    +1.12%