Make JSON/YAML output idiomatic with structured record values

[maebli]

Follow-up to #106, implementing the breaking cleanups it deferred. All changes are confined to the serialization layer; the parser API is untouched.

Changes

Structured record values. summary.records[].value was a table-style string like "(2850427)e-2[m³](Volume)". It is now split into structured fields derived from the processed data record header:

{
  "display": "(876543)e-3[m³](Volume)",
  "value": 876543.0,
  "exponent": -3,
  "unit": "m³",
  "quantity": "Volume",
  "data_information": "0,Inst,BCD 6-digit",
  "header_hex": "0B 13",
  "data_hex": "43 65 87"
}

The human-readable string is kept as display, and the table and CSV formats render from it, so their output is byte-for-byte unchanged. value, exponent, unit and quantity are omitted where they don't apply (dates, manufacturer-specific records).

Removed manufacturer_info. The top-level JSON/YAML key duplicated summary.manufacturer (code/name/website/description). Migration: read summary.manufacturer instead.

Hex strings for raw byte payloads. frame.data (wireless), data_records[].raw_bytes and manufacturer-specific record data now serialize as compact uppercase hex strings ("2F2F") instead of decimal byte arrays ([47, 47]). Implemented as serialize-only serde attributes backed by a new m_bus_core::serde_hex helper (no_std-compatible, collect_str-based), so Deserialize impls and the parser API are unchanged.

Versioning

This is a breaking change for JSON/YAML consumers: all workspace crates are bumped in lockstep from 0.1.4 to 0.2.0, and the migration is documented in CHANGELOG.md.

Verification

• cargo test --features std,decryption — 37 passed
• cargo test --features std — 34 passed
• cargo build (no default features), cargo check --workspace, clippy, fmt — clean
• Cross-format agreement test extended to assert the structured record fields match the normalized summary; new tests cover structured values, manufacturer_info removal, and hex payload serialization
• Verified end-to-end via the CLI: JSON/YAML show the new shape; table and CSV output unchanged

Note: cargo check -p m-bus-application-layer --features serde (without std) fails, but this is pre-existing on main and unrelated.