Feat: Add experimental packed representation

[mxsm]

Implements the experimental packed representation stage.

Scope:

• Adds experimental-packed feature.
• Adds a gated 24-byte packed::PackedCheetahString prototype.
• Adds packed layout/drop/clone/push tests.
• Adds unsafe proof checklist and cargo-fuzz targets.
• Does not wire packed representation into stable CheetahString.

Verification completed locally:

• cargo test --features experimental-packed
• cargo clippy --features experimental-packed -- -D warnings
• cargo check --manifest-path fuzz/Cargo.toml --bins
• final full verification matrix on integration branch

Closes #115.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 532d18f7-9acc-432e-beb4-d1ed789ed517

📥 Commits

Reviewing files that changed from the base of the PR and between 21aa0bf and 5a7abe0.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (31)

• .github/workflows/ci.yaml
• .github/workflows/release.yml
• Cargo.toml
• README.md
• bench-results/README.md
• benches/comprehensive.rs
• benches/layout.rs
• benches/mq_properties.rs
• benches/mq_remoting_header.rs
• benches/mq_topic.rs
• benches/mutation.rs
• benches/pattern.rs
• fuzz/Cargo.toml
• fuzz/fuzz_targets/packed_from_bytes.rs
• fuzz/fuzz_targets/packed_push_str.rs
• scripts/bench-all.ps1
• scripts/bench-all.sh
• src/bytes.rs
• src/cheetah_string.rs
• src/lib.rs
• src/packed.rs
• src/search.rs
• src/serde.rs
• src/simd.rs
• src/unsafe_proof.md
• tests/basic.rs
• tests/bytes.rs
• tests/layout_snapshot.rs
• tests/mutation.rs
• tests/packed.rs
• tests/search.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/pr-010-experimental-packed-repr

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