Add standalone manufacturer decoder crate

[maebli]

Adds a standalone no_std, allocation-free m-bus-manufacturer crate for maintainer-written vendor parsers. Each decoder is a plain Rust function plus a metadata table entry. Dispatch calls the first matching entry.

Checked readers return values with their byte ranges; small helpers turn them into named fields with scaling, units and labels. The synthetic example reads status, conditionally reads temperature, and reports hundredths of a degree Celsius. Errors preserve earlier fields and successful dispatch exposes leftover bytes.

No vendors or parser, CLI, output-format, or browser integrations are included yet.

Validation: all 11 crate/example tests pass, including automatic byte ranges and cursor rollback. Clippy with warnings denied, formatting, the embedded thumbv7em-none-eabi build, and Cargo package verification also pass.