RapidProto documentation

The user manual, one page per topic. The quick start is in the project README.

• arena.md - the arena decoder: accessors, the Arena, decode_owned, error handling
• streaming.md - the streaming decoder: field tags, the consumption patterns, aborting
• dumper.md - the --dump debug dumper: JSON-like inspection text, DumpOptions
• semantics.md - the shared rules: lifetimes, validation & trust, presence, open enums, duplicate fields
• using-both-models.md - both models in one TU; coexisting with protoc
• profiles.md - decode profiles (drop / raw) and unknown-field detection (arena)
• integration.md - the rapidprotoc CLI reference and the CMake helper
• osm-pbf.md - real-world walkthrough: OSM’s planet format with both models
• optimizations.md - how the decoders got fast: seven optimizations, re-enabled one at a time and measured
• benchmarks.md - the numbers and how to reproduce them

Contributor docs live at the repository root: architecture.md (internals, invariants, design rationale), CONTRIBUTING.md, SECURITY.md, and CHANGELOG.md.