Arrangement.Parse rejects a chart whose sections are separated by two blank lines or by a whitespace-only line #339

What's wrong

Arrangement.TryParse (Semantics.Music/Arrangement.cs:96) splits the text on exactly "\n\n". It then passes each block to Section.TryParse without trimming it. Section.TryParse requires the block's first character to be [ (Section.cs:116-118). Two layouts break this:

• Three newlines between sections. The next block starts with "\n", so it fails the [ check.
• A blank line holding a space or tab ("\n \n"). The split never happens, so two sections are read as one block.

Either way the whole parse fails.

Reproduced against HEAD 9c64b67, starting from a valid arrangement a with a Verse and a Chorus:

• Arrangement.Parse(a.ToString()) round-trips. CRLF input is also fine.
• Arrangement.Parse(a.ToString().Replace("\n\n", "\n\n\n")) throws FormatException.
• Arrangement.Parse(a.ToString().Replace("\n\n", "\n \n")) throws FormatException.

Why it matters

Arrangements are hand-written chart files. An extra blank line between sections, or an editor leaving trailing whitespace on the separator line, makes a valid chart unparseable. The error doesn't point at the cause.

Suggested fix

Split into lines and treat any run of empty or whitespace-only lines as one section separator. Alternatively, trim leading and trailing newlines from each block and drop blocks that are empty after trimming.

Acceptance criteria

• Both variants above parse equal to a.
• The existing arrangement round-trip tests still pass.