feat: add legacy-msrv feature for Rust 1.83+ compat

Adds a `legacy-msrv` feature flag that replaces the const
copy_from_slice (Rust 1.93+) with a manual byte loop, allowing
the crate to compile on older toolchains like Xtensa.

All other optimizations (sub-slice scan, zero fast path, decode
batch fill) work on 1.83 without the feature.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -15,6 +15,7 @@ categories = ["encoding", "no-std::no-alloc", "embedded"]
 
 [features]
 default = []
+legacy-msrv = []
 
 [lints.rust]
 unsafe_code = "forbid"