[Reese-max]

Summary

• serenity inflates every Discord gateway payload through flate2 with its default rust_backend (miniz_oxide + simd-adler32). That stack is implicated in the post-connect SIGSEGVs reported on x86_64 macOS release builds in SIGSEGV on x86_64 macOS after commit 843bb729 (observer/lobby Phase 1 scaffold) — Discord gateway crash loop #1531 (fault inside miniz_oxide::inflate in ShardRunner) and SIGSEGV in discord::Handler::handle_config_command after connect — 0.10.0-beta.4 release binary (macOS x86_64) #1535 (crash surfacing in handle_config_command during post-connect command sync).
• The discord feature now enables openab-core's flate2 dep with the zlib feature. any_c_zlib outranks rust_backend in flate2's backend selection, so all flate2 users in the build move to the OS zlib — removing the pure-Rust decompressor from the gateway hot path.
• Regression guard: tests/discord_gateway_zlib.rs asserts the manifest wiring and libz-sys lockfile resolution.
• A mechanical cargo fmt --all normalization commit plus a workspace_fmt_hygiene guard test are included because the tree had drifted from cargo fmt --all -- --check under current stable rustfmt (same drift fixed on the issue-1538 branch).

Refs #1535

Review Contract

Goal

Stop the post-connect SIGSEGVs on x86_64 macOS builds by removing miniz_oxide/simd-adler32 from serenity's gateway decompression path, via a dependency-feature change with no runtime code changes.

Non-goals

• Not claiming a proven root cause: SIGSEGV in discord::Handler::handle_config_command after connect — 0.10.0-beta.4 release binary (macOS x86_64) #1535's top frame is app code, so the decompression attribution is inferential (shared platform/version with SIGSEGV on x86_64 macOS after commit 843bb729 (observer/lobby Phase 1 scaffold) — Discord gateway crash loop #1531's confirmed miniz_oxide::inflate crash). If a crash persists on builds containing this change, it is a different defect.
• No changes to Discord app logic, command registration, or serialization.
• No simd-adler32 source modifications.
• Not attempting to remove miniz_oxide from the dependency graph entirely — other users (png/zip) still pull it; flate2 simply prefers the C backend at runtime.

Accepted Residual Risks

• Adds a C toolchain requirement for builds that enable discord/pre-seed (libz-sys vendors bundled zlib via cc when the system library is absent). Mitigation: all pinned CI/Docker build images (rust:1-bookworm, nightly-build) ship gcc and zlib1g.
• If the true SIGSEGV in discord::Handler::handle_config_command after connect — 0.10.0-beta.4 release binary (macOS x86_64) #1535 root cause is elsewhere, this change neither helps nor hurts; recovery procedure is continued triage on the issue with .ips crash logs.

Acceptance Criteria

• tests/discord_gateway_zlib.rs passes: discord feature includes dep:flate2 with zlib feature, and Cargo.lock resolves libz-sys.
• cargo fmt --all -- --check, cargo test --workspace, and cargo clippy --workspace --all-targets -- -D warnings pass on the candidate and on a clean detached replay (all verified).
• windows-gnu cross-check attempted: cannot run in this environment (pre-existing x86_64-w64-mingw32-gcc absence; ring build script fails identically at base). libz-sys supports mingw targets via bundled zlib.

Follow-ups

• Ask SIGSEGV in discord::Handler::handle_config_command after connect — 0.10.0-beta.4 release binary (macOS x86_64) #1535/SIGSEGV on x86_64 macOS after commit 843bb729 (observer/lobby Phase 1 scaffold) — Discord gateway crash loop #1531 reporters to confirm on the next beta build and attach .ips logs if crashes persist.
• Optionally add default-features = false to the flate2 dep to drop the dead rust_backend code when it is the sole declarer (cosmetic; serenity/png still request it in practice).

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[openab-app]

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