Add the editor: page view, cut editing, discard, levels, metadata

QGraphicsView for the viewport, with cuts and the content rectangle
manipulated by hit-testing in the view rather than as movable items —
the geometry is normalised, so what the screen shows and what the
renderer uses are the same numbers at a different zoom.

Cuts are edited as polylines: double-click adds one, drag moves it,
Ctrl-click inserts a vertex, right-click deletes a vertex or the whole
cut. That is how a straight cut becomes the stepped cut Engel needs.

Slice boundaries are drawn, not just cut lines, so a trim anomaly is
visible before export rather than after. Discarded slices are shaded.

Pages preview at 1800px regardless of source resolution, cached per
page, because re-reading a 4959x7017 vector render on every slider move
is unusable.

Autosave is debounced at 800ms and also fires on close.

Closes #12
Closes #14
Closes #15
Closes #16
Closes #17
Closes #18
Closes #19
Closes #20
Closes #26

tests/test_editor.py

@@ -0,0 +1,114 @@
+"""Runnable check that the editor builds and its edits reach project state.
+
+Runs offscreen, so it verifies wiring rather than appearance: that the widgets
+construct, that an edit changes the model, and that autosave and export work.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+import numpy as np  # noqa: E402
+import pymupdf  # noqa: E402
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from PySide6.QtWidgets import QApplication  # noqa: E402
+
+from noteman_slicer.detect import detect_page  # noqa: E402
+from noteman_slicer.editor import Editor  # noqa: E402
+from noteman_slicer.pdf import open_source, page_raster  # noqa: E402
+from noteman_slicer.project import Cut, Project, default_path  # noqa: E402
+
+W, H = 1200, 1600
+
+
+def _scan_pdf(path: Path) -> None:
+    art = np.full((H, W), 255, np.uint8)
+    for top in (300, 800):
+        art[top : top + 200, 100:104] = 0
+        for staff in (top, top + 140):
+            for i in range(5):
+                art[staff + i * 15 : staff + i * 15 + 2, 110:1100] = 0
+    pix = pymupdf.Pixmap(pymupdf.csGRAY, W, H, bytearray(art.tobytes()), False)
+    doc = pymupdf.open()
+    doc.new_page(width=595, height=842).insert_image(pymupdf.Rect(0, 0, 595, 842), pixmap=pix)
+    doc.save(path)
+
+
+def main() -> int:
+    tmp = Path(__file__).with_name("_tmp")
+    tmp.mkdir(exist_ok=True)
+    pdf = tmp / "scan.pdf"
+    _scan_pdf(pdf)
+
+    app = QApplication.instance() or QApplication(sys.argv[:1])
+    source = open_source(pdf)
+    gray = page_raster(source, 0)
+    project = Project.from_detection(pdf, [detect_page(gray)], [gray.shape[0]])
+
+    editor = Editor(source, project)
+    page = project.pages[0]
+
+    # Cuts.
+    before = page.slice_count
+    editor.view.selected_cut = page.add_cut(Cut.straight(0.5))
+    editor.view.redraw()
+    assert page.slice_count == before + 1
+
+    # A vertex turns a straight cut into a stepped one.
+    cut = page.cuts[editor.view.selected_cut]
+    cut.points.insert(1, (0.4, cut.y_at(0.4)))
+    cut.points[1] = (0.4, cut.points[1][1] + 0.03)
+    assert cut.straight_y is None, "the cut should no longer be straight"
+    editor.view.redraw()
+
+    # Discard.
+    editor.view.selected_slice = 1
+    was = page.discards[1]
+    editor.view.toggle_discard()
+    assert page.discards[1] != was
+
+    # Skew and levels reach the model and re-render without raising.
+    editor.skew.setValue(-1.4)
+    assert abs(page.skew + 1.4) < 1e-6
+    editor.black.setValue(40)
+    editor.white.setValue(210)
+    assert project.page_levels(0) == (40, 210)
+
+    # Metadata.
+    editor.metadata["title"].setText("Ketun joululaulu")
+    editor.metadata["composer"].setText("trad.")
+    assert project.metadata["title"] == "Ketun joululaulu"
+
+    # Content rectangle edits and reset.
+    page.content_rect = (0.05, 0.02, 0.95, 0.98)
+    assert project.page_content_rect(0) == (0.05, 0.02, 0.95, 0.98)
+    editor._reset_rect()
+    assert project.page_content_rect(0) == project.content_rect
+
+    # Autosave target, then a round-trip through disk.
+    editor._save()
+    saved = default_path(pdf)
+    assert saved.exists()
+    reloaded = Project.load(saved)
+    assert reloaded.metadata["title"] == "Ketun joululaulu"
+    assert reloaded.pages[0].skew == -1.4
+    assert reloaded.pages[0].levels == (40, 210)
+    assert [c.points for c in reloaded.pages[0].cuts] == [c.points for c in page.cuts]
+
+    editor.close()
+    source.close()
+    for f in (pdf, saved):
+        f.unlink()
+    tmp.rmdir()
+    print("ok")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())