Add code signing, notarization, and DMG packaging scripts

Enables distributing Galaxy.app to end users without Gatekeeper warnings.
Uses Samsung Developer ID Application certificate with hardened runtime.

Pipeline: cargo bundle → sign.sh → notarize.sh → package.sh → Galaxy.dmg

Entitlements:
- network.client (AWS Bedrock API calls)
- automation.apple-events (osascript for CLI install)
- cs.allow-unsigned-executable-memory (Metal shader compilation)

Orchestration scripts:
- build-debug.sh: full pipeline with debug build
- build-release.sh: full pipeline with release build

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

BuildSupport/Galaxy.entitlements

@@ -0,0 +1,13 @@
+<?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.network.client</key>
+    <true/>
+    <key>com.apple.security.automation.apple-events</key>
+    <true/>
+    <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+    <true/>
+</dict>
+</plist>