Pictures: duotone recolours, and layout/slide colour map overrides apply (#269)

Theme background pictures (Office 2007-era themes such as Treck) carry an
a:duotone recolour in phClr; DeckCraft read it but drew the picture
unchanged, so those backgrounds came out grey. The painter now resolves the
duotone's colours against the slide's colours and placeholder colour, and
the decoded-image cache applies them (luminance picks a colour between dark
and light) and keys on them.

Layouts and slides can replace the master's colour map
(p:clrMapOvr/a:overrideClrMapping, e.g. dark section layouts mapping bg1 to
dk1). They were read as the master's map; now Layout and Slide carry an
optional colour map, resolve::Ctx uses slide > layout > master, and the
writer keeps them.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/cmd/design.rs

@@ -483,6 +483,7 @@ fn insert_layout(s: &mut Session, p: &Value) -> Result<Value> {
                 background: None,
                 show_master_shapes: true,
                 preserve: true,
+                color_map: None,
                 raw_ext: None,
             },
         );