Fix default value encoding and bootstrap test failures

[dwerner]

Major Changes

1. ArenaDefault Trait Pattern

Adds a new pattern for constructing messages with default values in arena-allocated contexts.

Add ArenaDefault trait and derive macro

• Define ArenaDefault<'arena> trait with Builder associated type
• Auto-implement for types with arena lifetimes via #[derive(View)]
• Returns builder (not frozen view) for flexibility
• Only sets Plain/Required fields with explicit defaults
• Optional defaults to None, Repeated defaults to empty

Update test suite to use ArenaDefault

• Migrate all tests from manual construction to Type::arena_default(&arena)
• Remove explicit default value setters where not needed
• Cleaner, more consistent test code

Update documentation

• Add comprehensive README sections on ArenaDefault pattern
• Document builder pattern and arena allocation
• Add examples and best practices

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2. Project Reorganization (1 commit)

Reorganize benchmarks into dedicated benchmarks/ directory

• Move all benchmarks from defiant/benches/ to benchmarks/
• Update BENCHMARKS.md with new structure
• Cleaner separation of concerns

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3. Encoding Bug Fixes (2 commits)

Fix repeated message encoded_len

• Add missing key_len calculation for repeated message fields
• Was only calculating length_varint + content per message
• Forgot to multiply key_len by number of messages
• Fixes conformance tests: ValidDataRepeated.MESSAGE (proto2/proto3)
• Conformance: 1298 → 1300 successes (2 failures remaining)

Fix String/Bytes default value encoding

• String/Bytes with custom defaults now compare against actual value
• Only use .is_empty() optimization when default is actually empty
• Fixes check_default_values test: messages with defaults encode to 0 bytes
• Ensures protobuf spec compliance

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4. Option Type Conflict Resolution (3 commits)

Google's protobuf has a message type named Option that conflicted with Rust's std::option::Option<T>.

Resolve Option type name conflicts

• Code generator emits self::Option for user-defined types named "Option" or "Result"
• Derive macro filters out self::/super::/crate:: paths from Option detection
• Prevents "Failed to extract type from Option" panic

Regenerate protobuf bindings

• Generated code now uses self::Option qualifier
• Allows compilation of Google's protobuf descriptor types

Fix bootstrap test paths

• Update prost-types → defiant-types (package rename)
• Handle new CMake layout for conformance_test_runner
• Check both build/bin/ and build/ locations

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5. Supporting Improvements (2 commits)

Update test suite for fixed patterns

• Use ArenaDefault pattern consistently
• Add explicit type annotations for inference
• Fix method calls with as_ref() for borrowed types
• Remove unused imports

Add ArenaMap ordering traits and minor improvements

• Implement PartialOrd and Ord for ArenaMap
• Fix varint macro variable shadowing with closure pattern
• Update .gitignore for context snapshots and skill triggers

Files: tests/src/*.rs, defiant/src/arena.rs, defiant/src/encoding.rs, .gitignore

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Impact

• New Feature: ArenaDefault trait pattern for cleaner default value construction
• Bug Fixes: Encoding correctness for repeated messages and default values
• Reliability: All tests pass, improved conformance (1298 → 1300)
• Type Safety: User-defined types can be named "Option" without conflicts
• Documentation: Comprehensive README updates with examples
• Project Structure: Better organized benchmarks directory