Lataa xled 0.13.0

Behaviour changes
- Blank lines are rows. A blank line in the input is now a row with no fields, so row numbers match the file's lines (as --no-header always promised). On a file with blank lines, a script that addresses rows by number can select different rows than in 0.12.
- -i preserves the file. Rows a script does not change are written back byte for byte: line endings (LF, CRLF, CR), quoting, blank lines, the UTF-8 BOM and a missing final newline all survive. Previously every row was re-encoded (#27).
- round() edge cases: a value that rounds to zero is 0, not -0, and values too large for the requested decimals are no longer altered by float error.
- append is refused as out of scope, with the upstream route named.
New
- --diff prints one line per changed cell (C5 [price]: "$1,200" → "1200"). On its own it previews; with -i it records what was written. The REPL's preview uses it too.
- fixed(x, [decimals], [no_commas]), Excel's FIXED, for numbers as text with set decimals: fixed([amt], 2, true) gives 1234.50.
- Warnings when row 1 looks like a title above the real header, and when the file looks like it uses another delimiter (-d ';').
- Corrections that name the fix: header 1 → 1 header; [Name] → did you mean [name]; a bare header name → its bracketed form.
Fixes
- The skill and docs gave header N; the command is N header (#26).