feat(live): safe-at-rest gF filters + default-xfade dN prelude, and Slop-visuals scaffold

#21 gF footgun fixed and #13 xfade confirmed live this session:
- live/lib/prelude.tidal: nucleus of the ParVagues lib (#23). Two eval-blocks PLN
  can shadow into a running session (no reboot):
  * gF1/gF2/gF3 rebuilt on CORE hpf/lpf mapped so fader-at-rest (CC=0) == BYPASS.
    The old  (# djfbus N …)  referenced an UNregistered SuperDirt effect and rested
    at an extreme → muted the orbit with no recovery. New form structurally cannot
    mute: untouched = transparent, push fader = sweep in. PLN confirmed 'work great'.
  * d1..d12 = xfade N . (|< orbit N-1): every track LAUNCH crossfades 4 cycles AND
    keeps its orbit (old bare  xfade N  lost the orbit → wrong LCXL column/stem).
    Plus orbit-correct  jumpTo i  for hard cuts. PLN confirmed 'no more abrupt cut boom!'.
- visuals/slop/: scaffold for Kevin's Slopmotion — README (NOW=generate shareable
  MP4s, LATER=gig visual engine) + FEEDBACK.md log to hand back to him.

Not yet baked into BootTidal.hs — kept as evaluable prelude until fully road-tested.

live/lib/prelude.tidal

+-- ParVagues live prelude — TRY blocks (nucleus of the #23 lib)
+-- Open in Pulsar, eval a block (Ctrl-Enter inside the :{ … :}) to try it LIVE.
+-- These shadow the BootTidal.hs defs in the running session — no reboot needed.
+-- Once a block feels right, we bake it into BootTidal.hs (or load this file at boot).
+
+-- =====================================================================
+-- #21 — SAFE global filters (gF).  The footgun: `djfbus` is NOT a
+-- registered SuperDirt effect, and the fader CC rests at 0 → the old
+-- gF1/2/3 sent an extreme value into a missing bus → sound muted with
+-- no way back.  Fix: use CORE params (hpf/lpf, always available) mapped
+-- so REST (fader down = 0) == BYPASS.  Untouched = no effect, ever.
+-- Push a fader up → the filter sweeps in.  Cannot mute you.
+-- =====================================================================
+:{
+let gHPF ch = (# hpf (range 20 8000 ch))    -- fader up ⇒ high-pass climbs; rest(0)=hpf 20 ≈ open
+    gLPF ch = (# lpf (range 20000 180 ch))   -- fader up ⇒ low-pass closes; rest(0)=lpf 20k ≈ open
+    gF1 = gHPF "^49"    -- CC49 fader = high-pass build (tension riser)
+    gF2 = gLPF "^50"    -- CC50 fader = low-pass sweep (muffle/drop)
+    gF3 = id            -- CC51 spare, held safe (id = never touches sound)
+:}
+-- try:  d1 $ gF1 $ sound "bd*4"     then sweep fader 49 — no dropout at rest.
+-- (Old broken form was:  gF1 = (# djfbus 1 (range 0.05 0.95 "^49"))  — do not use.)
+
+
+-- =====================================================================
+-- #13 — DEFAULT-XFADE track launches.  Every dN now crossfades in over
+-- 4 cycles AND keeps its orbit (the old `xfade N` lost the orbit → sound
+-- landed on the wrong LCXL column / Ardour stem).  Fits the livemidi
+-- model: dN evals are track LAUNCHES, so auto-xfade has ~zero downside
+-- and makes every track→track move a clean crossfade with no code edit.
+--   d1 $ <track A>   … then later …   d1 $ <track B>   ⇒ B xfades over A.
+-- To hard-cut instead, use  jumpTo 1 $ …  (defined below, orbit-correct).
+-- =====================================================================
+:{
+let d1  = xfade 1  . (|< orbit 0)
+    d2  = xfade 2  . (|< orbit 1)
+    d3  = xfade 3  . (|< orbit 2)
+    d4  = xfade 4  . (|< orbit 3)
+    d5  = xfade 5  . (|< orbit 4)
+    d6  = xfade 6  . (|< orbit 5)
+    d7  = xfade 7  . (|< orbit 6)
+    d8  = xfade 8  . (|< orbit 7)
+    d9  = xfade 9  . (|< orbit 8)
+    d10 = xfade 10 . (|< orbit 9)
+    d11 = xfade 11 . (|< orbit 10)
+    d12 = xfade 12 . (|< orbit 11)
+    -- orbit-correct HARD CUT (when you DON'T want the 4-cycle blend):
+    jumpTo i = case i of
+        1  -> jump 1  . (|< orbit 0);   2  -> jump 2  . (|< orbit 1)
+        3  -> jump 3  . (|< orbit 2);   4  -> jump 4  . (|< orbit 3)
+        5  -> jump 5  . (|< orbit 4);   6  -> jump 6  . (|< orbit 5)
+        7  -> jump 7  . (|< orbit 6);   8  -> jump 8  . (|< orbit 7)
+        9  -> jump 9  . (|< orbit 8);   10 -> jump 10 . (|< orbit 9)
+        11 -> jump 11 . (|< orbit 10);  12 -> jump 12 . (|< orbit 11)
+        _  -> p i
+:}
+-- NOTE: while these xfade-dN are active, an *in-track tweak* also blends over
+-- 4 cycles (not instant). That's fine for launches; if you're iterating fast on
+-- one line, re-eval the plain BootTidal defs (or use  p N $ …  for an instant set).
+-- To make xfade length live-controllable later:  xfadeIn N <cycles> $ pat.

visuals/slop/FEEDBACK.md

+# Slopmotion — feedback log for Kevin
+
+Running log of reactions while using Slopmotion, to hand back to Kevin. Be specific
+and kind: what delighted, what confused, what broke, what we wished existed. Date every
+entry; note the version/build if known.
+
+Format per entry:
+- **[YYYY-MM-DD] [build?]** — context (what we were trying to do)
+  - 👍 loved: …
+  - 🤔 confusing: …
+  - 🐛 broke: … (repro steps)
+  - ✨ wish: …
+
+---
+
+<!-- entries below -->

visuals/slop/README.md

+# Slop visuals — ParVagues × Slopmotion (Kevin)
+
+**Slopmotion** is visual-generation software our friend **Kevin** is building. He's put
+a lot of work into it recently (new capabilities / capas). Two horizons for us:
+
+- **Short-term (NOW):** use it as a *generator* — produce easy, shareable **MP4** clips
+  for Instagram / socials from ParVagues material. Low commitment, high value.
+- **Long-term (LATER, not ready yet):** evaluate it as a **live gig visual engine**
+  (real-time backdrop, à la the HUD scene layer #22). Kevin's own read is it's not
+  there yet — so this stays exploratory, no gig dependency on it.
+
+## Our jobs here
+1. **Generate** — get Slopmotion running locally, feed it ParVagues audio/imagery,
+   render a first shareable MP4. Output clips land in this dir (`visuals/slop/out/`).
+2. **Give Kevin feedback** — capture reactions as we use it (see `FEEDBACK.md`).
+   He built the capas; concrete, kind, specific feedback is the gift back.
+3. **Assess as engine** (later) — does it fit the "perf-isolated backdrop" law
+   (never stalls audio/editor)? Compare with HUD Feature B (#22).
+
+## Pointers (fill in as we learn)
+- Slopmotion source / repo: **TODO — get from Kevin** (path or URL)
+- Install / run notes: TODO
+- Input formats it wants (audio? stems? images?): TODO
+- Export settings for Insta (aspect, codec, length): TODO — target H.264 mp4,
+  vertical 1080×1920 for stories/reels, ≤60s.
+
+Related: `../../live/` (source tracks), HUD scene backdrop (task #22),
+GLITCHWAVE source imagery.