refactor(fspy-shared): make the payload a borrowed view

[wan9chi]

Motivation

The end goal is a fully std-free preload, and the payload types were shaped against it: the channel configuration and every payload path crossed process boundaries as Box<IpcStr>, copied out of the deserialization buffer through the global allocator — on the attach path, under the loader lock on Windows.

Nothing needs to own payload fields. This makes Payload and EncodedPayload strictly borrowed views over storage their producer owns — the model the Windows preload already had with its 'static Detours page:

• ChannelConf<'a> borrows its paths; channel() returns only the Receiver, and Receiver::conf() derives the serializable configuration from receiver-owned C strings.
• decode_payload_from_env leaks its allocations into whichever allocator the caller passes — the allocator's lifetime bounds the payload's. The supervisor lends its session paths per spawn instead of cloning boxes.
• The unix preload attaches with one page-backed bump (fspy_nostd_alloc::page_bump()), held in ManuallyDrop from the start: the payload's process-lifetime storage lands at the bump's base, the sender's temporary path decode runs inside Bump::scoped so the same chunk is reused and reclaimed, and the bump is never dropped. One mapping for the whole attach (a second only if the payload outgrows the chunk), no global allocator, and the single unsafe — assume_process_lifetime — reasons directly against the ManuallyDrop declaration; scoped taking &mut self forces the promotion to happen before any temporary can exist. The copy out of the environment is deliberate — env memory is not stable storage. seccomp_payload stays owned until fspy_seccomp_unotify grows borrowed types.
• The Windows preload deserializes its payload zero-copy from the static page and forwards those original bytes to grandchildren instead of re-serializing per spawn.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.42%  [ -3.42% ..  +8.49%]  overhead   +50.59%
dynamic/access             change  -2.70%  [-21.30% ..  +6.92%]  overhead    +7.26%
dynamic/access-relative    change  +1.49%  [ -1.83% ..  +3.39%]  overhead   +48.50%
static/launch              change  +0.56%  [ -5.89% ..  +7.71%]  overhead  +144.78%
static/access              change  -0.11%  [ -1.17% ..  +0.84%]  overhead  +826.91%
static/access-relative     change  +0.02%  [ -0.91% ..  +0.96%]  overhead +1334.11%

macos

dynamic/launch             change  -0.73%  [ -5.22% ..  +3.69%]  overhead  +218.90%
dynamic/access             change  +0.34%  [ -4.44% ..  +5.86%]  overhead    +4.50%
dynamic/access-relative    change  -0.22%  [ -3.14% ..  +4.16%]  overhead  +243.82%

windows

dynamic/launch             change  -0.14%  [ -2.67% ..  +3.28%]  overhead   +26.81%
dynamic/access             change  +0.00%  [ -1.10% ..  +1.26%]  overhead    +1.42%
dynamic/access-relative    change  -0.17%  [ -3.70% ..  +3.93%]  overhead    +1.53%