no-std Step 1: swap std::error::Error and std::fmt to core:: in error/mod.rs

[Mec-iS]

Sub-task of #413.

What

Replace the two std:: imports in src/error/mod.rs with their core:: equivalents:

// Before
use std::error::Error;
use std::fmt;

// After
use core::error::Error;
use core::fmt;

core::error::Error is stable since Rust 1.81, within the declared MSRV 1.85.

The use std::error::Error inside #[cfg(test)] can remain std:: since tests always run with std.

Why first

This is the safest, zero-risk change and unblocks everything downstream by confirming core::error::Error compiles at the declared MSRV.

Acceptance

• use core::error::Error and use core::fmt in production code
• cargo test --all-features still passes
• cargo +1.85.0 build --all-features still passes (MSRV)

Estimated effort: ~30 min