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/appimage/build.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Wrap dist/DataTools/ (PyInstaller folder mode) into a distributable
+# AppImage.
+#
+# Usage:
+#   bash build/appimage/build.sh <version>
+#
+# Requires ``appimagetool`` on PATH (CI installs it; locally grab the
+# latest release from https://github.com/AppImage/AppImageKit/releases).
+#
+# Output: dist/DataTools-<version>-linux-x86_64.AppImage
+
+set -euo pipefail
+
+VERSION="${1:-0.0.0-dev}"
+DIST="dist/DataTools"
+OUT="dist/DataTools-${VERSION}-linux-x86_64.AppImage"
+
+if [[ ! -d "$DIST" ]]; then
+  echo "Error: $DIST not found. Run pyinstaller build/datatools.spec first." >&2
+  exit 1
+fi
+
+if ! command -v appimagetool >/dev/null 2>&1; then
+  echo "Error: appimagetool not on PATH. See build/appimage/build.sh header." >&2
+  exit 1
+fi
+
+# Lay out the AppDir.
+APPDIR="$(mktemp -d)/DataTools.AppDir"
+trap 'rm -rf "$(dirname -- "$APPDIR")"' EXIT
+mkdir -p "$APPDIR/usr/bin"
+
+cp -R "$DIST" "$APPDIR/usr/bin/"
+cp build/appimage/AppRun "$APPDIR/AppRun"
+chmod +x "$APPDIR/AppRun"
+cp build/appimage/datatools.desktop "$APPDIR/datatools.desktop"
+
+# Icon. AppImage requires a top-level <appname>.png next to the
+# .desktop. Use the build/icon.png if present, otherwise generate a
+# blank placeholder so the build doesn't fail on a fresh checkout.
+if [[ -f build/icon.png ]]; then
+  cp build/icon.png "$APPDIR/datatools.png"
+else
+  # 256x256 single-colour PNG via printf — appimagetool needs *some*
+  # icon present. Replace with a real 1024x1024 PNG before launch.
+  python3 - <<'PY'
+import struct, zlib, os
+def chunk(t, d): return struct.pack(">I", len(d)) + t + d + struct.pack(">I", zlib.crc32(t + d) & 0xffffffff)
+W = H = 256
+ihdr = struct.pack(">IIBBBBB", W, H, 8, 2, 0, 0, 0)  # 8-bit RGB
+raw = b"".join(b"\x00" + b"\x16\x19\x22" * W for _ in range(H))  # filter byte + dark pixels
+idat = zlib.compress(raw, 9)
+png = b"\x89PNG\r\n\x1a\n" + chunk(b"IHDR", ihdr) + chunk(b"IDAT", idat) + chunk(b"IEND", b"")
+out = os.environ["APPDIR"] + "/datatools.png"
+open(out, "wb").write(png)
+PY
+fi
+export APPDIR
+
+ARCH=x86_64 appimagetool "$APPDIR" "$OUT"
+echo "Built $OUT"