chore: declare rust-version = 1.87 MSRV and add deny(unsafe_code) lint gate

[e-desouza]

Why

The crate had no declared rust-version, making it unclear which Rust toolchain is required. The effective minimum is 1.87 (driven by usize::is_multiple_of, stable in 1.87, used in multiple files; edition = "2024" in xrpl-rust-macros requires ≥ 1.85).

Without a deny(unsafe_code) lint, there is no CI barrier against future introduction of unsafe blocks into a crate that currently contains none.

What changed

• Cargo.toml and xrpl-rust-macros/Cargo.toml: added rust-version = "1.87" to [package].
• Cargo.toml: added [lints.rust] unsafe_code = "forbid".
• src/lib.rs: added #![forbid(unsafe_code)] as first crate attribute.

How to validate

cargo build --release
cargo clippy --all-features -- -D warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.71%. Comparing base (c83df4f) to head (603fde9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #346   +/-   ##
=======================================
  Coverage   86.71%   86.71%           
=======================================
  Files         252      252           
  Lines       32630    32630           
=======================================
  Hits        28296    28296           
  Misses       4334     4334           

Flag	Coverage Δ
integration	71.32% <ø> (ø)
unit	87.33% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/lib.rs	0.00% <ø> (ø)

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[Copilot]

Pull request overview

This PR formalizes the repository’s minimum supported Rust version (MSRV) and adds a guardrail to prevent future introduction of unsafe code.

Changes:

• Declares rust-version = "1.87" for both the main crate and the xrpl-rust-macros proc-macro crate.
• Adds unsafe_code = "forbid" under [lints.rust] in Cargo.toml.
• Adds #![forbid(unsafe_code)] at the crate root in src/lib.rs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Cargo.toml	Declares MSRV 1.87 and forbids unsafe_code via Cargo lint configuration.
src/lib.rs	Enforces #![forbid(unsafe_code)] at the crate root.
xrpl-rust-macros/Cargo.toml	Declares MSRV 1.87 for the proc-macro crate.

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[e-desouza]

This PR is superseded by #352, which declares the same rust-version = "1.87" MSRV and [lints.rust] unsafe_code = "forbid" in both Cargo.toml and xrpl-rust-macros/Cargo.toml. Closing in favor of that PR.

[e-desouza]

Closing — superseded by #352.