Modernize to edition 2024, latest dependencies, workspace-inherited deps

[rudib]

Summary

• All crates on edition 2024, rust-version = "1.85", resolver = "3".
• Every dependency on its latest major and minor version: syn 3.0.6 (new major), bitvec 1.1.1, serde 1.0.229, quote 1.0.47, proc-macro2 1.0.107, error-chain 0.12.4.
• Dependencies and shared metadata (version, authors, edition, rust-version, license, repository) now live in [workspace.dependencies] / [workspace.package]; members use workspace = true.
• packed_struct and packed_struct_codegen bumped to 0.12.0 because of the MSRV raise.

Fixes the upgrade turned up

• Derive broken on edition 2024 (affects users). #[derive(PackedStruct)] generated { &<temp> }.pack()?. Edition 2024 drops tail-expression temporaries at the end of the block, so user crates on 2024 failed with E0716. It now emits (&<temp>).pack()?, which is correct in every edition (packed_struct_codegen/src/pack_codegen.rs).
• alloc feature never built on stable. Removed the nightly-only #![cfg_attr(feature="alloc", feature(alloc))] and added the missing Formatter import to the no_std prelude. Dropped "Requires nightly Rust" from the docs.
• serde: replaced serde_derive with serde's derive feature (use_serde = ["dep:serde", "serde/derive"]). std used serde/std, which silently turned serde on; it's now serde?/std. The implicit serde / serde_derive features are gone; use_serde is unchanged.
• bitvec 1.1 deprecations: shift_left/shift_right → shift_start/shift_end.
• Clippy: the CI clippy step already failed on current stable (15 errors in packed_struct_codegen, then more in packed_struct and the tests). Fixed mechanically: iN::MIN instead of min_value(), two dead fields removed, empty glob re-exports removed, #![allow(unused_imports)] on the internal prelude, and a check-cfg entry for a cfg that error_chain! expands to. The workspace now builds with zero warnings.
• Removed redundant extern crate / #[macro_use] lines; core::intrinsics::write_bytes → core::ptr::write_bytes.

Reviewer notes

• packed_struct_nostd_tests stays excluded from the workspace. The codegen emits ::std or ::core paths depending on its own std feature. A workspace-wide build unifies that feature, enabled by packed_struct_tests, and breaks the no_std crate. There's a comment explaining this in the root Cargo.toml.
• A no_std + alloc user crate still has to declare #[macro_use] extern crate alloc;, because the generated code relies on it. That's unchanged by this PR.
• CI: actions/checkout@v5, explicit dtolnay/rust-toolchain, feature-combination checks (--no-default-features, +alloc, --all-features), and a new MSRV 1.85 job.
• Cross matrix: replaced mips64-unknown-linux-gnuabi64 with s390x-unknown-linux-gnu. mips64 has been tier 3 since Rust 1.72 and has no prebuilt std, so cross build failed with "can't find crate for core". s390x keeps a big-endian target in the matrix on tier 2.

Testing

• cargo test --workspace: 117 passed, 0 failed (same as before), including doctests.
• cd packed_struct_nostd_tests && cargo test: passes.
• cargo clippy -p packed_struct_tests --all-targets -- -D warnings: clean.
• cargo check -p packed_struct with --no-default-features, --no-default-features --features alloc, --no-default-features --features use_serde, --features use_serde,byte_types_256 and --all-features: all pass without warnings.
• cargo +1.89 check --workspace --all-targets: passes. 1.85 was not installed locally, so the new CI job covers it.
• cargo package -p packed_struct_codegen: the packaged manifest resolves the inherited fields correctly.

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