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>

CLAUDE.md

@@ -45,7 +45,10 @@ about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns/file
 choosers from them and `src/cli/options.cpp` generates flags from them
 (`strip_height` → `--strip-height`). Never add effect-specific branches to the
 GUI or the parser — add a `ParamSpec` instead. A `Path` param names a file on
-disk; like a source path it is written relative to the project folder. Values
+disk; like a source path it is written relative to the project folder. A `Text`
+param is free-form text the user types, and a `Color` param is a colour written
+`#rrggbb` (`parseColor`/`formatColor`/`paramColor` in `src/core/params.hpp`),
+shown as a colour button in the interface. Values
 travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
@@ -96,7 +99,8 @@ is discarded rather than mixed into the new project (`dropLoadedImages_`).
 ### Two link targets, deliberately
 
 `effector_core` (static) holds image IO, params, effects, rng and links only
-`gdk-pixbuf-2.0` + Threads — **no gtkmm, no display**. The `effector` executable
+`gdk-pixbuf-2.0`, `pangocairo` (the text effect's glyphs; Pango and Cairo need
+no display) + Threads — **no gtkmm, no display**. The `effector` executable
 adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
 `src/core/` and `src/effects/`; the tests link the core alone and rely on this.
 
@@ -105,7 +109,10 @@ adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
 `Rng` (`src/core/rng.hpp`) is a hand-written xoshiro256\*\* rather than
 `<random>`, because libstdc++ distributions are not portable across
 implementations. Same sources + settings + seed must produce byte-identical
-output on any machine — the tests assert this. Effects must draw *every* random
+output on any machine — the tests assert this. The one thing outside this is the
+text effect's glyphs, which are whatever shapes the machine's fonts and
+fontconfig give: everything it is handed, the rng included, is still used the
+same way. Effects must draw *every* random
 value from the `Rng&` they are handed, and must not depend on thread scheduling:
 the scanlines effect first draws the whole strip plan serially from the rng,
 then copies pixels in parallel.