fix(ci): unbreak the self-hosted A1 runner (libmoq clippy + ffmpeg bindgen headers)

[kixelated]

Two independent failures break the Check job on the self-hosted A1 (arm64) runner introduced in #1775. Both are masked on GitHub-hosted Ubuntu and block trusted PRs (e.g. #1781).

1. libmoq clippy: platform-dependent cast

error: casting raw pointers to the same type and constness is unnecessary (`*const u8` -> `*const u8`)
   --> rs/libmoq/src/ffi.rs:231:35

cstr is *const c_char. c_char is u8 on arm64 Linux (no-op cast → unnecessary_cast under -D warnings) but i8 on x86/macOS (cast required, can't be removed). Fix: cstr.cast::<u8>() works on both and isn't flagged.

2. ffmpeg-sys-next bindgen can't find <errno.h>

.../ffmpeg-8.1-dev/include/libavutil/common.h:33:10: fatal error: 'errno.h' file not found
Unable to generate bindings: ClangDiagnostic(...)

The devshell exports LIBCLANG_PATH but not BINDGEN_EXTRA_CLANG_ARGS, so bindgen's libclang has no path to libc headers. GitHub-hosted Ubuntu has system glibc headers in /usr/include, which hid this; the A1 runner is headerless, so any cold-cache ffmpeg-sys-next build fails (a warm cache passes, which is why it looked intermittent). Fix: export BINDGEN_EXTRA_CLANG_ARGS from the cc wrapper's libc-cflags (Linux only; macOS uses the SDK).

Test plan

• cargo clippy -p libmoq --all-targets clean on macOS (c_char = i8)
• nix eval of both devShell shellHooks succeeds; Linux branch emits the BINDGEN_EXTRA_CLANG_ARGS export, Darwin branch unchanged
• Check green on the A1 runner (the real verification — neither issue reproduces on macOS)

(Written by Claude)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2c53df80-af12-4210-b1a8-449a7f818941

📥 Commits

Reviewing files that changed from the base of the PR and between e848e78 and deb0feb.

📒 Files selected for processing (2)

• flake.nix
• rs/libmoq/src/test.rs

✅ Files skipped from review due to trivial changes (1)

• rs/libmoq/src/test.rs

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Walkthrough

In rs/libmoq/src/ffi.rs, the parse_str function is updated to replace the pointer cast expression cstr as *const u8 with the equivalent method call cstr.cast::<u8>() when passing the input pointer to parse_slice. The same pointer cast idiom is updated across multiple test functions in rs/libmoq/src/test.rs for codec and path decoding.

In flake.nix, the devShells.default development shell configuration is updated to use Nix's built-in rustPlatform.bindgenHook instead of a manual shellHook. The bindgenHook automatically configures LIBCLANG_PATH and BINDGEN_EXTRA_CLANG_ARGS for bindgen header discovery, replacing the prior explicit shellHook definition.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: fixing two CI failures on the self-hosted A1 runner related to libmoq clippy warnings and ffmpeg bindgen headers.
Description check	✅ Passed	The description is well-related to the changeset, providing detailed context about both issues, platform-specific explanations, and test verification plans.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/libmoq-cchar-cast

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