backend, unify all descriptor emitters under --format dispatcher

[steven-varga]

Summary

Refactor h5cpp-compiler so that every backend descriptor emitter (HDF5, Protocol Buffers, JSON Schema, MessagePack, CBOR, BSON, Avro, RLP) plugs into a single unified CLI surface and dispatcher.

Current State

• Backends were developed serially in a single chain: feature, add h5:: attribute support to HDF5 backend #32 → feature, SQL schema backend emitter with PostgreSQL MySQL SQLite3 dialect support #33 → build, produce musl-static Linux binary for universal glibc compatibility #34 → refactor, emit visitor-only scatter/gather signatures #35 → #36 → #38.
• Binary backends introduced separate --json-out, --msgpack-out, etc. flags.
• tests/run_fixture.cmake has 8 copy-pasted golden-diff blocks (one per backend).
• Backend lanes cannot be developed or merged independently because they all edit the same central files.

Goal

1. Unified CLI: One --format [hdf5|protobuf|json|msgpack|cbor|bson|avro|rlp] flag + one -o <file> output path.
2. Unified dispatcher: A single switch (Format) in src/h5cpp.cpp so each backend only adds one case.
3. Generic test runner: run_fixture.cmake takes a BACKEND_FORMAT parameter; one diff block, not eight.
4. Embarrassingly parallel lanes: Each backend branches directly from staging with no commit-level dependencies.

Acceptance Criteria

• Foundation branch merged to staging containing shared attribute framework + unified dispatcher.
• PB lane (backend, unify all descriptor emitters under --format dispatcher #30→31) rebased onto foundation; uses --format protobuf.
• HDF5 lane (feature, add h5:: attribute support to HDF5 backend #32-specific) rebased onto foundation.
• JSON Schema lane (feature, SQL schema backend emitter with PostgreSQL MySQL SQLite3 dialect support #33) rebased onto foundation.
• MessagePack lane (build, produce musl-static Linux binary for universal glibc compatibility #34) rebased onto foundation.
• CBOR lane (refactor, emit visitor-only scatter/gather signatures #35) rebased onto foundation.
• BSON lane (#36) rebased onto foundation.
• Avro lane (#37, split from current 36-bson) rebased onto foundation.
• RLP lane (#38) rebased onto foundation.
• All existing tests pass; no regression in HDF5 output.