convert_recording: add --crf and --preset knobs

skills/recording.md

@@ -89,6 +89,13 @@ Needs a running TTS server at `--base-url` (default `http://127.0.0.1:8880/v1`).
     --apng <dasimgui>/doc/source/_static/tutorials/buttons.apng
 ```
 
+Mostly-static recordings (a plot with a brief gesture) land ~0.5–0.8 MB at the defaults. A
+**full-motion** clip (e.g. a realtime scroll, where every frame is a full-frame change) defeats
+H.264 inter-frame compression and runs several MB. Shrink it the lossless way — add
+`--preset veryslow` (better compression at the **same** quality), **not** by raising `--crf`
+(which trades away the quality the clip needs). `--crf` stays at the clean default `23`; both are
+knobs only for this case.
+
 Only the resulting `.mp4` is tracked. The `.apng`, `voiceover/*.wav`, `*.manifest.json`, `*.sidecar.json`, `*_music.wav`, and `*.mp4.ffmpeg.txt` are all intermediates and gitignored.
 
 ## Mental model

utils/convert_recording.das

@@ -36,6 +36,12 @@ struct Config {
     @clarg_doc = "Fade in/out length, milliseconds"
     fade_ms : int = 100
 
+    @clarg_doc = "libx264 quality (lower = better/larger). Default 23 - keep it here for excellent quality; shrink full-motion clips with --preset, not by raising this"
+    crf : int = 23
+
+    @clarg_doc = "libx264 preset (slower = smaller at the SAME quality). Default medium; use veryslow for full-motion clips (e.g. realtime scroll) to cut size without touching crf"
+    preset : string = "medium"
+
     @clarg_doc = "daScript root (find_sf2 + default daslang exe). Default: this exe's das-root"
     das_root : string
 
@@ -158,7 +164,7 @@ def main : int {
     var fargs <- ["ffmpeg", "-y", "-r", "{fps_eff}", "-i", cfg.apng, "-i", music_wav]
     fargs |> push_from(vo_inputs)
     fargs |> push_from(["-filter_complex", fg, "-map", "0:v", "-map", "[aout]",
-        "-c:v", "libx264", "-crf", "23", "-pix_fmt", "yuv420p",
+        "-c:v", "libx264", "-preset", cfg.preset, "-crf", "{cfg.crf}", "-pix_fmt", "yuv420p",
         "-movflags", "+faststart", "-shortest", out_mp4])
 
     // --- save the command, then run it ---