Arduino CLI: Strip UTF‐8 BOM from .ino before preprocessing

Summary

When a sketch .ino is saved as UTF-8 with BOM, the three BOM bytes (EF BB BF) reach the compiler and cause:

stray '357円' in program
stray '273円' in program
stray '277円' in program

This PR strips the BOM at read-time so the merged .cpp and any copied sources are clean.

Refs: #3015

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• The PR has no duplicates (please search among the Pull Requests before creating one)
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
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• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Bug fix — make the CLI robust to UTF-8 BOM at the start of .ino and additional files.

What is the current behavior?

• If a .ino is saved as UTF-8 with BOM, the BOM bytes are preserved into the merged .cpp, leading to compiler errors (stray '357円' / '273円' / '277円').
• This matches IDE issue arduino/arduino-ide#2752 and appears "random" to users because some editors silently add a BOM; a blank line after an initial block comment makes it easy to reproduce.

What is the new behavior?

• On reading sketch sources:
  • Strip a leading UTF-8 BOM before merging .ino files.
  • Strip a leading UTF-8 BOM when copying additional files.
• Result: BOM-prefixed sketches compile successfully. No behavior change for normal UTF-8 (no BOM) files.

Implementation notes

• Added helper:

func stripUTF8BOM(b []byte) []byte {
 if len(b) >= 3 && b[0] == 0xEF && b[1] == 0xBB && b[2] == 0xBF {
 return b[3:]
 }
 return b
}

• Applied in:
  • internal/arduino/builder/sketch.go → sketchMergeSources() (via getSource(...))
  • internal/arduino/builder/sketch.go → sketchCopyAdditionalFiles(...)

Test plan (manual)

1. Create a minimal sketch:

/* test */
int x = 42;
void setup(){ Serial.begin(9600); }
void loop(){ Serial.println(x); delay(1000); }

2. Save with BOM (VS Code → Save with Encoding → UTF-8 with BOM).
3. Compile:

arduino-cli compile -b arduino:avr:uno <sketch-folder>

Before this patch: fails with:

stray '357円' in program
stray '273円' in program
stray '277円' in program

After this patch: succeeds.

Control: Save as UTF-8 (no BOM) → succeeds (unchanged).

(Optional follow-up): add an automated test by placing a BOM-prefixed .ino in testdata and asserting the merged output compiles.

Does this PR introduce a breaking change?

No. The change only strips a BOM if present; no impact on existing UTF-8 (no BOM) files or other encodings.

Other information

• The issue was reported in the IDE repo, but the root cause is in the CLI merge/preprocess path. Fixing it here resolves the problem for the IDE once it bundles a CLI containing this patch.
• Performance/overhead is negligible (constant-time 3-byte check per file).