docs: settle decoder edge cases and encoder output for 4.2

[johannschopplich]

The 4.2 minor release settles the decoder cases the ports read differently and removes the encoders' remaining choices. Every decoder change concerns input no conforming 4.1 encoder emits, and the encoder changes are tightenings, so 4.1 output decodes unchanged.

• Decoding – a line of tabs is not blank, raw control characters inside quotes decode as themselves, a header-shaped line without a colon is a scalar line, an indented first line is over-indented, a hyphen marks a list item only at item depth, an empty key token is "" while an empty field entry is an error, whitespace before a nested field group is an error, and a " opens a quoted span anywhere in a line.
• Non-strict mode – a decoder applies only the leniencies the spec names, or errors where they are optional. Depth jumps gain one: the first line's depth becomes the scope's content depth.
• Encoding – shortest round-trip digits, no quotes unless required, and primitive list items quoted by the document delimiter.

Stacked on #65. Four new cases fail the reference until toon-format/toon#357 lands. The date line still shows 4.1's; it is set at release.