feat(sigsafe): add thin C string byte iteration

[wan9chi]

Motivation

Thin C strings currently expose only a raw-pointer constructor and a whole-string count operation. Allocation-free callers that need to inspect bytes must duplicate unsafe NUL scanning, and callers that have already checked pointer nullability cannot preserve that proof.

This adds CStr<Thin>::bytes() and a reusable fused byte iterator, plus from_non_null so checked pointers stay non-null through construction. from_ptr remains as the unchecked raw-pointer adapter, and count reuses the byte iterator.

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[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.78%  [ -4.75% ..  +6.06%]  overhead   +46.81%
dynamic/access             change  -0.50%  [ -3.67% ..  +3.45%]  overhead    +4.74%
dynamic/access-relative    change  -0.22%  [-10.66% ..  +4.73%]  overhead   +40.00%
static/launch              change  +0.40%  [ -4.44% ..  +6.63%]  overhead  +150.17%
static/access              change  +0.14%  [ -3.88% ..  +6.70%]  overhead  +716.14%
static/access-relative     change  -1.06%  [ -5.65% ..  +4.29%]  overhead  +874.76%

macos

dynamic/launch             change  -0.38%  [ -4.42% ..  +3.28%]  overhead  +224.87%
dynamic/access             change  +0.01%  [ -3.42% ..  +3.56%]  overhead    +6.00%
dynamic/access-relative    change  -0.32%  [ -6.42% ..  +7.66%]  overhead  +273.58%

windows

dynamic/launch             change  +0.38%  [ -4.47% ..  +5.60%]  overhead   +26.38%
dynamic/access             change  +0.54%  [ -1.40% ..  +6.73%]  overhead    +1.60%
dynamic/access-relative    change  +0.33%  [ -6.56% ..  +4.83%]  overhead    +1.42%