feat(u): add u slot subcommand for Solidity storage slot computation

[bit2swaz]

Closes #3417.

What this does

Adds a u slot subcommand (alias u s) to the u CLI. Given a Solidity storage layout type and an ordered list of keys, it prints the storage slot as a 0x-prefixed 32-byte hex value to stdout.

u slot "mapping(uint256 => mapping(uint256 => uint256)[])" 100 1 123
# 0x00a9b48fe93e5d10ebc2d9021d1477088c6292bf047876944343f57fdf3f0467

Keys go outermost to innermost. The output is bare (no label), so it pipes into other tools without any stripping.

Implementation

• New module tools/u/src/slot.rs with three private functions: parse_layout (wraps the bare type in a dummy state variable so syn-solidity can parse it), parse_mapping_key (maps a Solidity key type and string value to MappingKey), and build_slot (walks the type tree consuming keys front-to-back, allocating each Slot node in a typed_arena::Arena to satisfy the borrow-based lifetime constraints).
• Reuses the existing solidity-slot library unchanged.
• Wired into main.rs as Cmd::Slot with visible alias s.
• Added syn-solidity and typed-arena to workspace dependencies.

Test coverage

14 unit tests covering: nested mapping + array layout parsing, all supported key types (uint256, uint64, bytes32, string), rejection of unsupported types and malformed inputs, the known-answer vector from the issue, too-few and too-many keys, a single mapping, a single dynamic array, and a bytes32 key.

Checklist

• cargo clippy -p u --all-targets passes
• cargo fmt -p u produces no diff
• cargo test -p u passes (14/14)
• Known-answer vector confirmed end-to-end via CLI
• Positional args (not flags), per maintainer feedback
• Lives in existing tools/u crate, no new crate

[vercel]

@bit2swaz is attempting to deploy a commit to the unionbuild Team on Vercel.

A member of the Team first needs to authorize it.

[bit2swaz]

@benluelo lmk if any changes are needed :)