ci: release workflow drafting a GitHub Release, and docs/releasing.md

A push to the release branch (or workflow_dispatch) builds the macOS universal dmg and CLI zip,
the Linux tar.gz and AppImage and the Windows zip, with the version from Cargo.toml. The last
job checks the set, writes SHA256SUMS.txt and notes saying how the macOS app was signed, and
creates or updates a draft release; it never publishes. Off the release branch it only uploads
the files as a run artifact (dry run). macOS signing and notarization use the APPLE_* secrets
when they exist and fall back to ad-hoc signing (#72).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

.github/release-notes.md

@@ -0,0 +1,27 @@
+FieldCraft is open-source CAE pre-processing in Rust: it turns geometry into analysis-ready
+finite element models, writes Abaqus, Nastran and LS-DYNA decks, and solves linear static and
+modal problems with its own solver. AI agents can drive it over MCP (`fieldcraft-cli mcp`).
+[README](https://github.com/jyviko/fieldcraft#readme) ·
+[roadmap](https://github.com/jyviko/fieldcraft/blob/main/ROADMAP.md) ·
+[issues](https://github.com/jyviko/fieldcraft/issues)
+
+**Which file?**
+- **macOS 11+** (Apple silicon and Intel): open `fieldcraft-{version}-macos-universal.dmg` and
+  drag `FieldCraft.app` onto Applications. The command-line tool is in
+  `fieldcraft-cli-{version}-macos-universal.zip`.
+- **Linux** (x86_64, glibc 2.35+: Ubuntu 22.04, Debian 12, Fedora 36 or newer):
+  `fieldcraft-{version}-linux-x86_64.AppImage` (`chmod +x`, then run it), or unpack
+  `fieldcraft-{version}-linux-x86_64.tar.gz` and run `bin/fieldcraft` (`bin/fieldcraft-cli` is the
+  command-line tool).
+- **Windows 10+** (x64): unzip `fieldcraft-{version}-windows-x64.zip` and run `fieldcraft.exe`
+  (`fieldcraft-cli.exe` is the command-line tool).
+- `SHA256SUMS.txt`: `sha256sum -c SHA256SUMS.txt --ignore-missing` (macOS:
+  `shasum -a 256 -c SHA256SUMS.txt --ignore-missing`).
+
+To start with a sample model, run the app with `--sample cantilever` (macOS:
+`/Applications/FieldCraft.app/Contents/MacOS/FieldCraft --sample cantilever`).
+
+**Signing.** {macos_signing} The Linux and Windows builds are not signed; on Windows, SmartScreen
+may say "Windows protected your PC": choose **More info**, then **Run anyway**.
+
+Every package carries `LICENSE-MIT`, `LICENSE-APACHE`, `NOTICE` and `THIRD-PARTY-LICENSES.txt`.