feat: add contextual qjson errors

[membphis]

Summary

• add qjson_format_error and qjson_doc_last_error_offset to expose contextual error messages through the C ABI
• track access failure offsets on documents and format Lua parse/access errors with snippets, expected types, and byte offsets
• improve EOF/unclosed-container parse reporting for eager grammar/depth errors while preserving unclosed-string EOF errors

Closes #124

Tests

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

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 8622b3e1-0a68-4e96-875b-71d8b2e5c56d

📥 Commits

Reviewing files that changed from the base of the PR and between 613b5aa and ba7f46d.

📒 Files selected for processing (11)

• include/qjson.h
• lua/qjson.lua
• lua/qjson/lib.lua
• lua/qjson/table.lua
• src/doc.rs
• src/error.rs
• src/ffi.rs
• tests/ffi_parse_error_offsets.rs
• tests/lua/basic_spec.lua
• tests/lua/lazy_table_spec.lua
• tests/lua/options_spec.lua

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-124-error-context

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