Replace the handoff notes with a durable spec and ADRs

The handoff was written as a message to relay information; several
decisions lived only there. Split into permanent homes:

- docs/spec.md — scope, geometry model, pipeline order, detection,
  levels, editor, project file, markers, bundle format, noteman's
  changes, reference values
- ADR 0002 — raster only in release 1; SVG slices measured at 40x WebP
  naive, 2.6x with a bounding-box cull, deferred on risk not size
- ADR 0003 — lossless WebP with levels and 16-level alpha; every lossy
  option and alternative format measured larger for line art
- ADR 0004 — detection proposes, the human disposes; no unattended mode
- ADR 0005 — PyMuPDF for all PDF access, accepting AGPL
- ADR 0006 — systems are found by vertical brackets, not row-darkness
  gaps, which miscount every page of a 6-voice score

Also from testing against the hardest score in the repertoire: scanned
PDFs carry their scan as an embedded image and must be extracted at
native resolution rather than re-rendered at 600 DPI, and per-page
deskew is mandatory (skew varies -2.6 to +1.2 within one PDF).

CONTEXT.md

@@ -32,10 +32,11 @@ pre-sets it on a page's top and bottom slice when they contain no system.
 _Avoid_: delete, skip, exclude
 
 **Slice image**:
-The rendered artifact of a slice. From a raster source: lossless WebP, RGB pure
-black, `alpha = 255 − luminance`, width capped at 1920px — paper is transparency,
-ink is alpha. From a vector source: SVG with text converted to paths. Both are
-display-ready as produced; nothing downstream reprocesses them.
+The rendered artifact of a slice: lossless WebP, RGB pure black,
+`alpha = 255 − luminance`, width capped at 1920px — paper is transparency, ink is
+alpha. Display-ready as produced; nothing downstream reprocesses it. An SVG form
+for vector sources is designed but deferred, which is why the geometry model is
+renderer-agnostic.
 _Avoid_: PNG, page image, tile
 
 **Marker**: