refactor(fspy): rename native IPC string types

[wan9chi]

Motivation

The existing native_str, NativeStr, and NativePath names describe their platform representation but not their role. These types are specifically the lossless native strings and paths carried across fspy and task IPC boundaries.

Rename the crate to fspy_ipc_str and the types to IpcStr and IpcPath so their purpose is explicit before adding direct conversions to the no-std path types.

This is a mechanical rename with no wire-format or behavior changes.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.67%  [-14.72% .. +10.31%]  overhead   +49.70%
dynamic/access             change  +1.34%  [ -9.67% .. +21.66%]  overhead    +7.58%
dynamic/access-relative    change  -0.74%  [-18.65% .. +14.09%]  overhead   +48.15%
static/launch              change  +1.55%  [-11.01% .. +15.18%]  overhead  +147.88%
static/access              change  -0.26%  [ -8.60% ..  +7.46%]  overhead  +951.20%
static/access-relative     change  -0.77%  [ -4.16% ..  +3.43%]  overhead +1377.10%

macos

dynamic/launch             change  +0.23%  [ -3.11% ..  +4.04%]  overhead  +219.46%
dynamic/access             change  -2.58%  [-10.77% .. +11.00%]  overhead    +3.93%
dynamic/access-relative    change  +0.22%  [ -5.74% ..  +9.82%]  overhead  +234.15%

windows

dynamic/launch             change  +1.36%  [-13.56% .. +13.65%]  overhead   +25.80%
dynamic/access             change  -0.71%  [-17.13% ..  +9.80%]  overhead    +0.87%
dynamic/access-relative    change  -1.19%  [-21.16% ..  +2.41%]  overhead    +1.37%