Cut 1.6.0: shapes, connectors, text, selection by area, and a grid

[marcosqlbi]

Pull request 7 of the 1.6.0 plan in docs/design-objects.md: the release cut. The five
features are on main — the grid (#134), selection by area (#135), text labels (#136),
shapes and the Insert tab (#137), and connectors (#139) — and none of them touched the
prose, so this is where a person is told what they got. #138, the native editable export,
must merge before this one: the release notes and the guide both say shapes and labels
leave as objects in a deck and a PDF, which is true once it lands.

VersionPrefix goes 1.5.2 → 1.6.0, so CHANGELOG.md gains the matching section and the
Release notes check applies to this pull request. ./scripts/release-notes.ps1 -Mode Verify -Version 1.6.0 passes, and dotnet build Whiteboard.sln -c Release is clean.

What it affects

• CHANGELOG.md — one ## 1.6.0 - 17 September 2026 section with three entries, which
  is the shape of 1.5.0 (3), 1.5.1 (1) and 1.5.2 (2): the Insert tab with shapes,
  connectors and text, selection by area, and the grid. The toolbar preference is one
  sentence inside the first entry rather than a fourth entry, because it is where somebody
  meets it. The first entry also carries the one thing a person deciding whether to upgrade
  has to know: a board holding any of the new objects does not open in 1.5.x.
• README.md — three bullets in Included in the application, seven rows in
  Controls, the Preferences lists in the bullet and in the table, and the containers
  paragraph, which now says shapes and labels are containers and that a shape is picked up
  by its outline.
• site/guide.html — a new Shapes, connectors, and text section after Text and
  code, a paragraph on area selection under Containers, one sentence on the grid under
  Navigation, and the Insert tab named in the command-strip section.
• site/shortcuts.html — Insert, Lasso and Grid rows, the rubber band and Ctrl-to-add,
  and the Alt-mnemonic row extended with Alt+I, Edit L, View G, and Insert T.
• site/index.html — two feature tiles, shapes/connectors/text and select by area.
• docs/decisions.md — decision 31, why design objects are ordinary board objects,
  why a shape is taken by its outline, and why the grid is an application preference. The
  rest stays in docs/design-objects.md, whose status line is now shipped in 1.6.0.
• TODO.md — the current version, which still said 1.3.0, and what 1.6.0 left out.

Readings taken, since nobody could be asked

• site/compare.html is untouched. It has no row saying Whiteboard lacks shapes, text,
  a grid or multi-select; the nearest, Ink and objects, is a statement about strokes
  travelling with a container and is still true. The brief said to change such cells and
  nothing else, so nothing else changed.
• site/faq.html is untouched. No answer on it became wrong.
• Architecture in README.md is untouched. The Core bullet names no types — it
  says "world geometry, camera math, retained board objects, commands, hit testing" — so
  there was no list for ShapeGeometry, ConnectorGeometry and RotatedRectangle to join,
  and the brief said to leave it alone in that case.
• Escape is an edited row, not a new one, in both the README table and the shortcut
  list: Escape already had one, and a second would contradict it.
• site/changelog.html is untouched, as it is generated from CHANGELOG.md at
  deployment.

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