scuffle hardcodes *const i8 for C strings, so it only compiles where c_char is signed

Start by reviewing the affected signatures in src/scope.rs, src/snapshot.rs, src/property_group.rs, src/instance.rs, src/service.rs, and src/value.rs, plus the casts in src/scf.rs and src/value.rs. Compare them with the existing libc::c_char usage and run cargo build for the aarch64 target or stub configuration; done means the crate builds without the reported type mismatches.

Description

scuffle wraps libscf-sys, which correctly declares its non-illumos stubs in
terms of c_char. scuffle itself hardcodes i8 in nine function signatures
and four explicit casts, so the two only agree on platforms where c_char is
signed. On aarch64, where c_char is unsigned, the crate fails to compile with
17 errors pointing in both directions:

error[E0308]: mismatched types
   --> scuffle-0.1.0/src/snapshot.rs:53:35
    |
 53 |     .scf_get_snapshot(name.as_c_str().as_ptr(), handle.as_mut_ptr())
    |      ---------------- ^^^^^^^^^^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`

error[E0308]: mismatched types
   --> scuffle-0.1.0/src/value.rs:373:17
    |
371 |     libscf_sys::scf_handle_decorate(
    |     ------------------------------- arguments to this function are incorrect
373 |         decoration,
    |         ^^^^^^^^^^ expected `*const u8`, found `*const i8`

Both directions appear because CStr::as_ptr() returns *const c_char
(unsigned here) while scuffle's own signatures say i8, and libscf-sys's
stubs say c_char while scuffle passes i8 into them.

The affected sites are:

src/scope.rs:37             name: *const i8
src/snapshot.rs:74          name: *const i8
src/property_group.rs:219   name: *const i8
src/instance.rs:143,157,168 name: *const i8
src/service.rs:88,102       name: *const i8
src/value.rs:368            decoration: *const i8
src/scf.rs:129,152,250      .cast::<i8>()
src/value.rs:731            .cast::<i8>()

The crate already uses libc::c_char correctly elsewhere — src/iter.rs,
src/libscf_sys_supplemental.rs and src/property_group.rs:458 all declare
*mut libc::c_char — so this looks like an oversight rather than a decision.

I understand SMF is illumos-only and that these code paths are stubs that
unimplemented!() off illumos. The point is narrower: the stubs exist
specifically so that higher-level crates can build on other platforms (the
libscf-sys module comment says exactly that), and on aarch64 they do not.
This blocks building dendrite and omicron on aarch64 Linux.

Steps to reproduce

1. On an aarch64 Linux host: cargo build any crate depending on scuffle 0.1.0.
2. 17 error[E0308] mismatches in scuffle.

Expected result

The crate compiles on any platform where the stubs are used, matching the stated
purpose of libscf-sys's stub module.

Actual result

error: could not compile 'scuffle' (lib) due to 17 previous errors

Suggested fix

Replace the hardcoded i8 with libc::c_char in the nine signatures and four
casts listed above. On x86_64 this is a no-op, since c_char is i8 there.

Environment

scuffle 0.1.0 (crates.io)
via oxidecomputer/omicron @ df990b05, oxidecomputer/dendrite @ 04a52a32
platform aarch64-unknown-linux-gnu, Ubuntu 24.04
rust 1.98.1 (omicron), 1.90.0 (dendrite)

Disclosure: this issue was investigated and written up with AI assistance
(Claude). Everything in it was measured rather than inferred — the timings,
error output, version numbers and reproduction steps are all from real runs on
real hardware, and where a fix is suggested it is one I am actually running. I
have read it through before filing. Happy to clarify anything or test a patch.