Add fuzz targets for custom Block, Mark, and Content serde

[gvonness-apolitical]

Summary

• Add block_parser fuzz target: exercises custom Block::deserialize with all 20+ block type dispatch paths
• Add mark_parser fuzz target: exercises custom Mark::deserialize including both string and object mark parsing
• Add content_parser fuzz target: exercises full Content deserialization pipeline with nested blocks and marks
• Register all 3 new targets in fuzz/Cargo.toml

The hand-written serde impls from PR #87 are the most complex recent addition. These targets increase confidence against malformed input causing panics or unexpected behavior.

Test plan

• cargo check in fuzz/ directory compiles all 5 targets
• Main workspace: cargo build/test/clippy --workspace --all-features all pass
• CI passes all 10 jobs

Usage

cargo +nightly fuzz run block_parser
cargo +nightly fuzz run mark_parser
cargo +nightly fuzz run content_parser

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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