Feat/test coverage (6/8)

[bnomei]

Linked Issue

Closes #

Description

Expand test coverage with fuzz targets, a benchmark harness, and boundary/spec edge-case tests to catch regressions early. This adds encode/decode fuzzing, criterion benchmarks, and tests for max depth, large arrays, long strings, empty structures, and spec edge cases. Coverage stats are captured to track progress. This is PR 6 of 8 in the intended sequence (1-8).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Add fuzz targets for encode/decode and a benchmark harness for encode/decode.
• Add boundary tests for max depth, large arrays, long strings, and empty structures.
• Add spec edge-case tests for keys, delimiters, quoting, and numeric rules.

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version:

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Tests run: cargo test
Fuzzing: short runs completed (nightly)
Benchmarks: harness added; not run yet

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

Migration path:

Additional Context

Coverage snapshot (cargo llvm-cov):

• Default features: Lines 60.63%, Regions 61.34%, Functions 62.74%
• Library-only: Lines 87.37%, Regions 86.05%, Functions 90.07%

Branch: feat/test-coverage
Intended PR order: 1-8 (this is PR 6 of 8)