build: wire desktop-bundle pipeline (CI matrix + per-platform installers)

Stand up the seamless-download path for non-technical buyers:

* .github/workflows/build.yml — matrix CI (mac/win/linux) that builds
  PyInstaller bundles and packages them per platform on tag push,
  attaching the resulting installers to a GitHub Release.
* build/installer.iss — Inno Setup script for the Windows installer
  (per-user install, optional desktop shortcut, runs on finish).
* build/macos/build_dmg.sh — wraps DataTools.app into a .dmg with a
  drag-to-/Applications layout.
* build/appimage/{AppRun,datatools.desktop,build.sh} — AppImage recipe.
* src/__init__.py — single source of truth for __version__; the spec
  reads it (was hardcoded), CI passes it through to all packagers.

Buyer download path now lives in the top-level README. Per-build
README documents the Phase 2 step (signing/notarization) that needs
the owner's Apple Developer + Windows code-signing credentials —
those are intentionally not in CI yet because they require setup
outside this repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/macos/build_dmg.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Wrap dist/DataTools.app into a distributable .dmg.
+#
+# Usage:
+#   bash build/macos/build_dmg.sh <version>
+#
+# Run after ``pyinstaller build/datatools.spec --clean --noconfirm``
+# has produced ``dist/DataTools.app``. The output DMG goes to
+# ``dist/DataTools-<version>-mac.dmg``.
+#
+# Code signing + notarization happen separately (see build/README.md
+# "Signing"). This script only handles the packaging step.
+
+set -euo pipefail
+
+VERSION="${1:-0.0.0-dev}"
+APP="dist/DataTools.app"
+DMG="dist/DataTools-${VERSION}-mac.dmg"
+
+if [[ ! -d "$APP" ]]; then
+  echo "Error: $APP not found. Run pyinstaller build/datatools.spec first." >&2
+  exit 1
+fi
+
+# Drag-target convenience: a /Applications symlink inside the DMG so
+# the buyer can drag the app icon to it without leaving the DMG.
+STAGE="$(mktemp -d)"
+trap 'rm -rf "$STAGE"' EXIT
+
+cp -R "$APP" "$STAGE/"
+ln -s /Applications "$STAGE/Applications"
+
+# UDZO = compressed read-only DMG, the standard distribution format.
+hdiutil create \
+  -volname "DataTools" \
+  -srcfolder "$STAGE" \
+  -ov \
+  -format UDZO \
+  "$DMG"
+
+echo "Built $DMG"