fix: support current Rust toolchains

[chmanie]

Summary

• migrate package-local build-std configuration to the current nightly immediate-abort panic strategy
• keep the workspace manifest compatible with stable Cargo used by CI and releases
• fix the current-nightly MIDI packet Clippy lint

Supersedes #611.

Verification

• latest nightly: formatting, firmware/libfp Clippy, libfp tests, firmware release build, and UF2 generation
• latest stable: formatting, firmware/libfp Clippy, libfp tests, and firmware release build

Hardware test

• Flash firmware and confirm the firmware boots normally

[ArthurGibert]

Migrates panic_immediate_abort to the syntax current nightly requires, and swaps chunks_exact(4) → as_chunks::<4>() in midi.rs. Confirmed necessary (the old config fails to compile core on current nightly — reproduced directly) and confirmed correct (as_chunks behavior is identical to chunks_exact, zero-cost per asm diff; the new alloc build-std addition is genuinely required by postcard-bindgen-core, not scope creep).

Worth addressing before merge: the new config hard-fails on stable Cargo, where the old one silently no-op'd and still produced a working (larger) binary — reproduced directly both ways. There's no rust-toolchain.toml anywhere in the repo pinning nightly for the faderpunk package, and gen-bindings.sh has to say cargo +nightly explicitly, implying bare cargo resolves to stable here. That risks breaking the documented cd faderpunk && cargo build --release / ./build-uf2.sh workflow for anyone without a personal rustup override already set. Suggest adding a rust-toolchain.toml pinning nightly for faderpunk/.

Worth knowing, not blocking:

• Because .cargo/config.toml is scoped to faderpunk/, CI/release builds (which run from repo root) never see this fix at all — this PR only affects the local build-uf2.sh dev path.
• panic = "immediate-abort" makes ordinary app panics compile to silent traps with no defmt/RTT output — pre-existing, not introduced here, but worth a code comment given how much it undermines local debugging.
• A release-build rust-lld alignment warning showed up during testing — almost certainly the same pre-existing issue #614 fixes independently, not new here.