feat: add encode_number_precision

[membphis]

Summary

• add qjson.encode_number_precision() with module-level precision state and cached float format strings
• apply configured precision to float encoding while preserving integer and lazy fast paths
• document the supported cjson migration API and add Lua coverage for bounds, errors, and lazy proxy behavior

Test Plan

• PATH="$HOME/.luarocks/bin:$PATH" make test
• cargo test --release --no-default-features
• cargo test --features test-panic --release
• make lint

Closes #121

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: f629c38e-ae1d-4632-b7a5-8d8959f45922

📥 Commits

Reviewing files that changed from the base of the PR and between 9d7a9f3 and 9ac7209.

📒 Files selected for processing (4)

• docs/migrating-from-cjson.md
• lua/qjson.lua
• lua/qjson/table.lua
• tests/lua/encode_number_precision_spec.lua

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/121-encode-number-precision

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