Feat: Prepare v2.0.0 breaking API cleanup and release

[mxsm]

Closes #128.

Summary:

• Bump crate metadata and docs to v2.0.0.
• Remove deprecated safe byte constructors from CheetahString while keeping checked and explicit unsafe constructors.
• Make CheetahString::from_string use owned mutable storage by default.
• Keep fuzz lock metadata aligned with v2.0.0.

Verification:

• cargo fmt -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --no-default-features
• cargo test --no-default-features --features "serde bytes simd"
• cargo package --allow-dirty
• cargo check --manifest-path fuzz\Cargo.toml --bins

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 73224dc5-fbf4-4fa6-bdbe-38f67e10441a

📥 Commits

Reviewing files that changed from the base of the PR and between b0fbb68 and 60161f2.

⛔ Files ignored due to path filters (1)

• fuzz/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (7)

• .github/workflows/release.yml
• Cargo.toml
• README.md
• bench-results/README.md
• src/cheetah_string.rs
• src/lib.rs
• tests/mutation.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat-2.0.0-breaking-release

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