feat: match upstream multiple ino ordering

[zackees]

Summary

• Match upstream multiple-.ino handling by selecting a primary sketch before concatenating additional tabs.
• Prefer Arduino-style <sketch>/<sketch>.ino, support PlatformIO-style src/main.ino, and fall back to a file containing setup() or loop().
• Sort additional .ino tabs case-insensitively with exact filename order as a stable tie breaker.
• Document the Arduino CLI and PlatformIO behavior used for the compatibility decision.

Closes #647

Investigation

• Arduino CLI documents preprocessing as primary file matching the sketch folder first, followed by remaining .ino/.pde files alphabetically, then adding .cpp to the generated file.
• PlatformIO's InoToCPPConverter scans top-level PROJECT_SRC_DIR/*.ino/*.pde, moves a file containing setup() or loop() to the front when present, and names the generated output from the selected main .ino path.
• fbuild now combines those rules for Arduino sketch folders and PlatformIO src/ projects.

Tests

• soldr cargo fmt
• soldr cargo test -p fbuild-build source_scanner
• soldr cargo clippy -p fbuild-build --all-targets -- -D warnings
• soldr cargo test -p fbuild-build

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 6c87833c-64fa-42cb-9671-2071d5dce7df

📥 Commits

Reviewing files that changed from the base of the PR and between 7c5b3eb and df190dc.

📒 Files selected for processing (2)

• crates/fbuild-build/src/source_scanner.rs
• docs/reference/platformio-compatibility.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/647-multiple-ino-order

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