feat: match Arduino CLI and PlatformIO behavior for multiple ino files #647

Context

The current .ino preprocessing path supports multiple .ino files by sorting them and concatenating them before preprocessing. The generated output name is based on the first .ino stem.

This may or may not match Arduino CLI and PlatformIO. fbuild should intentionally match upstream behavior instead of relying on an assumed ordering rule.

Relevant area:

• crates/fbuild-build/src/source_scanner.rs

Proposal

Investigate how Arduino CLI and PlatformIO handle sketches with multiple .ino files, then make fbuild match that behavior.

The investigation should cover:

• File ordering rules.
• Whether the primary sketch file is treated specially.
• How generated .cpp output is named.
• Whether tab/file order differs between Arduino CLI and PlatformIO.
• Edge cases when file names differ only by case on Windows/macOS.

Acceptance criteria

• The issue documents the observed Arduino CLI behavior.
• The issue documents the observed PlatformIO behavior.
• fbuild matches the chosen upstream-compatible behavior.
• Tests cover multiple .ino files, including the primary sketch file and additional .ino tabs.
• Behavior is documented for users.

Open questions

• If Arduino CLI and PlatformIO differ, which one should fbuild prioritize, or should this become a compatibility mode?