refactor(fspy-shm): adopt the no-std OsCStr API

[wan9chi]

Motivation

With the backing path supplied by the caller (#667), nothing left in fspy_shm needs std: the remaining OsStr/CString/io::Error uses were adapters at the API boundary. Dropping them makes the crate usable from the preload contexts where std is not dependable, matching fspy_nostd's guarantees.

fspy_shm is no_std now: create/open/remove take fspy_nostd::OsCStr paths (bytes on Unix, wide units on Windows), create takes NonZeroUsize, and every function returns fspy_nostd::Result with raw OS error codes.

Supporting surface, each piece consumed immediately:

• fspy_nostd gains the OsCStr alias and a Fat→Thin downgrade.
• fspy_nostd_alloc compiles on every platform now that only arena and fs are unix-specific; CString is generic over the code unit, renames its byte accessors to unit accessors, gains a checked from_vec_with_nul, and an OsCString alias mirrors OsCStr.
• fspy_shared owns the OsStr boundary: it encodes paths into OsCString, keeps one in its keeper, and maps raw OS errors back to io::Error.

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[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.45%  [-11.69% .. +11.86%]  overhead   +52.44%
dynamic/access             change  -0.13%  [-16.33% .. +27.79%]  overhead    +9.55%
dynamic/access-relative    change  -1.52%  [-16.72% ..  +6.29%]  overhead   +47.37%
static/launch              change  +1.21%  [-10.19% .. +16.23%]  overhead  +150.64%
static/access              change  -0.77%  [ -9.36% .. +10.63%]  overhead  +958.61%
static/access-relative     change  +0.08%  [ -4.22% ..  +4.29%]  overhead +1287.92%

macos

dynamic/launch             change  +0.19%  [ -4.17% ..  +5.10%]  overhead  +236.76%
dynamic/access             change  -1.03%  [-59.84% .. +10.11%]  overhead    +0.37%
dynamic/access-relative    change  +0.40%  [ -8.18% .. +86.07%]  overhead  +263.40%

windows

dynamic/launch             change  -0.15%  [ -3.96% ..  +3.45%]  overhead   +26.75%
dynamic/access             change  +0.00%  [ -2.50% ..  +4.51%]  overhead    +1.87%
dynamic/access-relative    change  +0.18%  [ -1.58% ..  +1.81%]  overhead    +1.58%