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).

docs/adr/0003-lossless-webp-with-levels-and-alpha-quantisation.md

@@ -0,0 +1,58 @@
+# Lossless WebP, with levels and alpha quantised to 16 levels
+
+Slice images are encoded as **lossless WebP**, with the levels adjustment applied
+and the alpha channel quantised to 16 levels. About 7 KB per slice, ~450 KB for a
+65-system song. Every lossy option and every alternative format measured
+*larger* for this content, which is the opposite of the usual intuition — hence
+this record.
+
+## The measurement
+
+20 slices of one real song, levels applied throughout, relative to plain lossless
+WebP:
+
+| | vs baseline | |
+|---|---|---|
+| **WebP lossless + alpha quantised to 16** | **68%** | chosen |
+| AVIF q60 | 90% | lossy, for 10% |
+| WebP lossless | 100% | baseline |
+| WebP lossy q85 (alpha) | 107% | |
+| AVIF q85 | 114% | |
+| JXL lossless | 130–133% | |
+| WebP lossy q85 (opaque ink-on-white) | 158% | |
+| PNG grayscale + alpha | 165% | |
+| AVIF lossless | 188% | |
+
+Separately, before levels: applying levels alone takes 338 KB → 211 KB, a 38%
+reduction.
+
+## Four results that contradict an instinct
+
+- **Lossy is bigger than lossless here.** Not a quality problem — the measured
+  difference between q85 and lossless is max 12/255, mean 0.33, i.e. invisible.
+  Lossy VP8 simply spends more bits on sharp black/white edges than VP8L's
+  palette and predictor transforms do, and notation is nothing but sharp edges.
+  The "q85 looks fine" intuition comes from photographs and inverts here.
+- **AVIF and JXL both lose**, AVIF lossless by nearly 2×. Their lossless modes
+  are afterthoughts on photo codecs. WebP's VP8L is close to purpose-built for
+  flat two-tone line art — sheet music is the content type it is best at. JXL
+  additionally has no path forward in Chrome.
+- **Alpha costs nothing.** Opaque ink-on-white and black-plus-alpha are within
+  0.1% at lossless, so paper-tint removal and future non-rectangular slices are
+  free.
+- **Levels is the single biggest lever** — 38%, as a side effect of a control
+  that exists for quality reasons anyway. Pushing the white point below the
+  paper's luminance sets vast regions to exactly `alpha = 0`, which costs almost
+  nothing to encode.
+
+Alpha quantisation to 16 levels is imperceptible: antialiased edges span 2–3 px
+at 1920, and 16 steps across that is below notice. 8 levels starts to gamble on
+thin strokes.
+
+## Rejected as not worth it
+
+- **Encoder effort tuning** — Pillow's `method=6` buys 3% and a dependency.
+- **`alpha_quality=60`** — 24%, for less control than quantisation gives.
+- **Grayscale WebP** — no such mode exists. It wouldn't help anyway: the RGB
+  channels are constant black and compress to nearly nothing, so alpha is the
+  entire payload.