Warn on non-CRLF line endings in ADIF multiline fields

[Copilot]

ADIF MultilineString fields must use CRLF for embedded line endings per spec section III.A. The checker now flags lone CR/LF in :M fields as warnings while continuing normal validation.

• Validation path

  • Added a warning-capable validator path (validateADIFWithWarnings) that preserves existing fatal validation behavior.
  • Kept validateADIF as the error-only wrapper to avoid changing existing call semantics.

• Multiline line-ending checks (:M only)

  • Scans multiline field payloads and emits warnings for:
    • \r not followed by \n
    • standalone \n
  • Accepts proper \r\n sequences without warning.

• CLI output behavior

  • main now prints accumulated warnings to stderr.
  • Validation still succeeds when input is otherwise well-formed.

• Coverage/docs

  • Added tests for CRLF (no warning), LF-only (warning), CR-only (warning), and non-M fields (no warning).
  • Updated README checks list to document multiline line-ending warnings.

warnings, err := validateADIFWithWarnings(data)
if err != nil {
	fmt.Fprintf(os.Stderr, "invalid ADIF: %v\n", err)
	os.Exit(1)
}
for _, warning := range warnings {
	fmt.Fprintf(os.Stderr, "warning: %s\n", warning)
}