v1.6.1 — styles tells the truth

A defect sweep on 1.6.0. No schema change (schemaVersion stays 1), no
operation-shape change, no new surface.

Fixed

assemblash styles now lists dropShadow. 1.6.0 rendered the effect
but the CLI's discovery listing still named only the five pre-1.6 effects,
so an agent reading styles concluded drop shadows were unsupported. The
listing is now derived from the Effect enum in core instead of a literal
array in the CLI, a new effect variant fails to compile until it is given
its line, and the regression test asserts every variant by an exhaustive
match. The dropShadow entry documents its four fields (dx, dy,
blur, color), notes that dx/dy at 0 is a glow, and that an
#rrggbbaa alpha sets flood opacity.
The add-text --text and set --text help no longer claim `\n`
is a line break. A backslash-n typed at the shell is stored as the two
characters and renders as such; only --text-file produces a real line
break. The help now says the text is taken verbatim and points at
--text-file for a line break. Behaviour is unchanged.

Upgrade: 1.6.1 is a drop-in replacement for 1.6.0. Documents written by
1.6.1 open unchanged in 1.6.0 and in every build back to 1.0.