build: add the macOS gate, darwin artifacts, and a Homebrew formula

CI runs fmt/clippy/test plus a real alpine boot on a macos-26 runner
with libkrun from the krun tap; the release workflow ships a signed
darwin-arm64 tarball beside the snaps. The formula installs from the
tap, carries the runtime kegs (libkrun, e2fsprogs, libarchive), and
codesigns the binary with the hypervisor entitlement — Hypervisor
.framework grants VM creation only to entitled binaries on current
macOS, the same requirement krunkit's formula signs for.

packaging/macos/ecrou.entitlements

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0"><dict>
+<key>com.apple.security.hypervisor</key><true/>
+</dict></plist>