Feat: prelude and new words

docs/prelude.md

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+# The Prelude
+
+When you define a word in a step, it becomes available to all steps. But when you close and reopen the project, the dictionary is empty again. Words defined in steps only exist after those steps run.
+
+The **prelude** solves this. It's a project-wide script that runs automatically when playback starts and when you load a project.
+
+## Accessing the Prelude
+
+Press `d` to open the prelude editor. Press `Esc` to save and evaluate. Press `D` (Shift+d) to re-evaluate the prelude without opening the editor.
+
+## What It's For
+
+Define words that should be available everywhere, always:
+
+```forth
+: kick "kick" s 0.9 gain . ;
+: hat "hat" s 0.4 gain . ;
+: bass "saw" s 0.7 gain 200 lpf . ;
+```
+
+Now every step in your project can use `kick`, `hat`, and `bass` from the first beat.
+
+## When It Runs
+
+The prelude evaluates:
+
+1. When you press Space to start playback (if stopped)
+2. When you load a project
+3. When you press `D` manually
+
+It does not run on every step, only once at these moments. This makes it ideal for setup code: word definitions, initial variable values, seed resets.
+
+## Practical Example
+
+A prelude for a techno project:
+
+```forth
+: k "kick" s 1.2 attack . ;
+: sn "snare" s 0.6 gain 0.02 attack . ;
+: hh "hat" s 0.3 gain 8000 hpf . ;
+: sub "sine" s 0.8 gain 150 lpf . ;
+0 seed
+```
+
+Step scripts become trivial:
+
+```forth
+c1 note k sub
+```
+
+The sound design lives in the prelude. Steps focus on rhythm and melody.