Release prep: 0.3.0 and linux-riscv64 release job (#108)

* Release prep: bump to 0.3.0; add linux-riscv64 release job

- release.yml: new linux-riscv64 job (cross-compiled on ubuntu-24.04 against ports.ubuntu.com
  riscv64 libraries, tar.gz only, CLI smoke-tested under qemu-riscv64), ported from GridCraft;
  the release job needs it so SHA256SUMS covers its tarball.
- packaging/linux/package.sh: CROSS_ARCH / CROSS_TARGET / CROSS_COMPILE / EMULATOR support.
- Version 0.2.0 -> 0.3.0 (cargo xtask version set), Cargo.lock refreshed.
- docs/releasing.md, docs/hardware-parity.md: list the riscv64 tarball.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* CI: tell actionlint about the org's github-ubuntu-16core runner label

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -6,7 +6,7 @@ exclude = ["plugins"]
 # default-members = ["crates/*", "apps/figcraft", "apps/figcraft-cli", "xtask"]
 
 [workspace.package]
-version = "0.2.0"
+version = "0.3.0"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 rust-version = "1.95"