Add a curated list of 10 sampling classics as built-in demo sources — fuel for
demoing the Foundry, precomputing a fixed validation corpus, and iterating on
loop/chop quality (#19 autotune input).

WHY: we need a stable, pedagogically-diverse set to validate the finder across
material types instead of ad-hoc URLs. The set spans the axes that stress the
finder differently: canonical drum BREAKS (Amen, Funky Drummer, Apache — the
gold standards; if the rubric can't nail these it's wrong), a BASS-defining
groove (Chic – Good Times), VOCAL/no-drums (Loituma), ORCHESTRAL/no-drums edge
case (Mozart 40 — exercises the no-drums grid fallback), a full clean POP mix
(Rickroll), hip-hop sample-collage (Grandmaster Flash Wheels of Steel, Humpty
Dance), and a drum+vocal-stab combo (Lyn Collins – Think).

WHAT:
- demos.json — authored data (slug/title/year/source/why/expect/tags). Each
  entry flags drums-presence because the finder's grid-from-drums path depends
  on it (no-drums → bass/first-stem fallback, per the recall notes).
- engine/demos.py — loader (load / by_slug / source_of).
- foundry.py "demos" command — list, or precompute via --catch <slug> / --all
  (+ --sep to separate). This batch path is exactly what #19 autotune consumes.
- server.py — GET /api/demos; /api/fetch now accepts "ytsearch1:" queries too.
- ui/index.html — a 'sampling classics' quick-pick strip (click → fills the URL,
  tooltip shows why/expect).

HONESTY ON LINKS: only Loituma (from the repo's own test fixture) and Rickroll
(universally known id) ship as verified watch URLs (✓). The rest use
"ytsearch1:" queries (≈) that resolve to the top hit at fetch time — so we never
ship a guessed video id that silently breaks. yt-dlp accepts both forms.

VALIDATION: foundry demos lists 10 (✓/≈ marked); /api/demos serves the JSON;
UI strip renders; loader resolves source_of('amen-break').