Add a text effect

Writes a line or several over its input in a chosen family, weight, size
and colour, optionally on a box filled with a colour or with a picture
from disk. Like the blur and the overlay it needs no sources of its own,
so it can lead a chain; given sources it fills the canvas from a randomly
chosen one first.

Two param types rather than effect-specific branches in the front-ends:
Text, free-form text the user types, and Color, a colour written
"#rrggbb". Both travel as strings, so the interface gets an entry and a
colour button, the command line gets a flag (a colour that does not parse
is refused there rather than drawn as black), and a project keeps them
across a round trip.

The glyphs come from Pango, which draws through Cairo without a display,
so the core stays headless — but they are the one thing here that is not
byte-identical across machines: a machine without the chosen family
renders the substitute its fontconfig picks. Everything the effect is
handed, the rng included, is used exactly as every other effect uses it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/registry.cpp

@@ -5,6 +5,7 @@
 #include "effects/monochrome.hpp"
 #include "effects/overlay.hpp"
 #include "effects/scanlines.hpp"
+#include "effects/text.hpp"
 
 namespace effector {
 
@@ -16,6 +17,7 @@ const std::vector<EffectPtr>& effects() {
         std::make_shared<GrainEffect>(),
         std::make_shared<MonochromeEffect>(),
         std::make_shared<OverlayEffect>(),
+        std::make_shared<TextEffect>(),
     };
     return registry;
 }