feat: support PlatformIO check tool metadata

[zackees]

Summary

• add check_tool metadata to BoardConfig
• preserve check_tool from PlatformIO env config without feeding it into build flags
• expose check_tool_ini_value() for generated platformio.ini emission and document the key

Closes #645

Tests

• soldr cargo test -p fbuild-config check_tool
• soldr cargo test -p fbuild-config
• soldr cargo clippy -p fbuild-config --all-targets -- -D warnings

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: c1ab5286-3bf8-478a-ae4a-bbd9e20dcaf6

📥 Commits

Reviewing files that changed from the base of the PR and between 13a9bbd and 8bb5d31.

📒 Files selected for processing (7)

• crates/fbuild-config/src/board/loaders.rs
• crates/fbuild-config/src/board/methods.rs
• crates/fbuild-config/src/board/tests.rs
• crates/fbuild-config/src/board/types.rs
• crates/fbuild-config/src/ini_parser/mod.rs
• crates/fbuild-config/src/ini_parser/tests.rs
• docs/reference/platformio-ini.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/645-check-tool

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