feat(core): add timeline — DAW-style keyframe widget

Timeline widget with:
- Header: play/pause + reset + duration drag + loop checkbox
- Ruler: 0.5s ticks, scrub via click+drag
- Tracks: horizontal rows with diamond-shaped draggable keyframes
- Playhead: vertical primary_light line + ruler triangle marker

State and types:
- Keyframe { time, value, ease }
- Track { name, vector<Keyframe> }
- TimelineState { tracks, current_time, duration, playing, loop }

Pure functions:
- track_value_at(track, t): interp between keys, ease applied via the
  destination keyframe (Maya/AfterEffects convention)
- timeline_update(state, dt): advance current_time, wrap or saturate

Render with fn_tokens for visual coherence with the rest of the design
system. Keys are sorted by time on every changed frame to keep order
consistent during drag.

cpp/functions/core/timeline.h

@@ -0,0 +1,67 @@
+#pragma once
+
+// timeline — widget tipo DAW: tracks horizontales con keyframes interpolados,
+// scrub y play/pause. Sirve para animar valores escalares (uniforms shader,
+// parametros UI, etc) a lo largo del tiempo.
+//
+// Estado puro (TimelineState) + funciones puras de interpolacion
+// (track_value_at) + render impuro (timeline_widget).
+//
+// Uso:
+//   static fn::TimelineState tl;
+//   tl.tracks.push_back({"hue", {{0,0}, {2,1}, {4,0}}});
+//   tl.duration = 4.0f;
+//
+//   fn::timeline_update(tl, dt);
+//   float h = fn::track_value_at(tl.tracks[0], tl.current_time);
+//   fn::timeline_widget("##tl", tl);
+
+#include "core/tween_curves.h"
+
+#include <imgui.h>
+#include <string>
+#include <vector>
+
+namespace fn {
+
+struct Keyframe {
+    float time;
+    float value;
+    tween::Ease ease = tween::Ease::Linear;
+};
+
+struct Track {
+    std::string name;
+    std::vector<Keyframe> keys;
+};
+
+struct TimelineState {
+    std::vector<Track> tracks;
+    float current_time = 0.0f;
+    float duration     = 5.0f;
+    bool  playing      = false;
+    bool  loop         = true;
+};
+
+// --- Pure -------------------------------------------------------------------
+
+// Interpola el valor de `track` en el tiempo `t`. Asume keys ordenadas por
+// time. Si `t` cae antes del primer keyframe devuelve el value del primero;
+// si cae despues del ultimo, devuelve el value del ultimo. Entre keyframes
+// usa el ease de la SEGUNDA key (la "curva entrante" hasta esa key).
+float track_value_at(const Track& track, float t);
+
+// --- Update -----------------------------------------------------------------
+
+// Avanza current_time si playing. Si loop=true hace wrap; si no, satura en
+// duration y pone playing=false al llegar.
+void timeline_update(TimelineState& s, float dt);
+
+// --- Render -----------------------------------------------------------------
+
+// Widget completo: cabecera con play/pause + tiempo, ruler con scrub, y un
+// panel por track con keyframes draggable. Devuelve true si el usuario hizo
+// algun cambio (drag de keyframe, scrub, play/pause).
+bool timeline_widget(const char* id, TimelineState& s, ImVec2 size = ImVec2(-1, 200));
+
+} // namespace fn