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/0006-systems-are-found-by-brackets-not-row-gaps.md

@@ -0,0 +1,67 @@
+# Systems are found by vertical brackets, not by row-darkness gaps
+
+System detection anchors on the **vertical bracket / barline** that spans a
+system's staves, and uses the row-darkness profile only to expand each anchor to
+its ink extent. The obvious approach — find gaps in the row-darkness profile and
+cut in the middle of them — does not work on multi-voice choral scores, which is
+most of the corpus.
+
+## Why the obvious approach fails
+
+A row-darkness profile cannot distinguish an **inter-staff** gap from an
+**inter-system** gap. In a 6-voice closed score, one system is six staves joined
+by a bracket, and the gaps between those six staves look exactly like the gap
+between two systems — only smaller, and not reliably so.
+
+Measured on *Ketun joululaulu*, a 12-page 6-voice arrangement and the hardest
+score in the repertoire:
+
+- On page 2's first system, staff gaps run ~47px against a ~211px system gap. A
+  merge threshold tuned there works.
+- On the same page's second system the lyrics fill the inter-staff gaps, so the
+  ratios invert and the same threshold merges the wrong things.
+
+Result across all 12 pages, row-profile-only versus bracket-anchored:
+
+| | bracket-anchored | row-profile only |
+|---|---|---|
+| systems per page | 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 1 | 10, 6, 7, 7, 7, 8, 8, 5, 4, 5, 8, 4 |
+
+The bracket-anchored counts match the score. The row-profile counts are wrong on
+every page, and wrong by a different amount each time — so no threshold fixes
+them.
+
+## The algorithm
+
+1. **Deskew per page.** Projection-profile variance sweep over ±5°. Measured skew
+   on this song ranges −2.6° to +1.2° *between pages of the same PDF*, so per-page
+   is not optional.
+2. **Find anchors.** Binarise, then morphological open with a tall thin kernel
+   (height ≈ 3% of the page) so only long vertical strokes survive. Take
+   connected components taller than 4% of the page; walk them tallest-first,
+   keeping each one whose y-extent doesn't overlap an already-kept anchor. Each
+   surviving stroke is one system.
+3. **Expand to ink.** Compute the row-darkness profile on a despeckled copy, take
+   its ink runs, and assign each run to the nearest anchor by centre distance. A
+   system's extent is the union of its runs.
+4. **Place cuts** at the midpoint between consecutive systems' ink extents.
+
+Step 3 is what makes this work rather than the bracket alone: a bracket stops at
+the last staff line, but the slice must include the **lyrics below it**. On page
+2, system 1's bracket spans 177–994 while its true ink extent is 179–1071 — the
+77px difference is the bottom voice's lyric line, which the bracket misses
+entirely and the row profile finds.
+
+## Consequences
+
+- Detection needs both signals. Neither the column pass nor the row pass is
+  sufficient alone, so `detect.py` computes both.
+- **Scores without brackets** — single-staff melodies, lead sheets — have no
+  anchors, and fall back to row-profile runs. That fallback is the *only* correct
+  behaviour there, since every ink run genuinely is its own system.
+- Bar numbers printed above a system (this score uses 11, 16, …) sit in their own
+  ink run and get absorbed into the nearest system by step 3. That is right: they
+  belong to the system they label.
+- A page number can be absorbed the same way if its darkness clears the profile
+  threshold, inflating the last system's extent. The content rectangle and the
+  bottom discard slice both prevent this; don't rely on the threshold.