feat: add debug-only FORGE_RUNTIME_BINARY runtime override

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Summary

Add a debug-only escape hatch (FORGE_RUNTIME_BINARY) so developers can point the macOS menu bar app at a locally built forge3 instead of the managed runtime store, and make the app runnable unbundled by skipping Sparkle when there is no .app bundle.

Changes

• Add DeveloperRuntimeOverride, which resolves FORGE_RUNTIME_BINARY into an InstalledRuntime in DEBUG builds only
• Add DeveloperOverrideRuntimeInstaller, a RuntimeInstalling wrapper that short-circuits download, staging, and store validation when an override is active
• Wire the wrapper into AppDelegate runtime setup via wrapIfOverridden
• Make the Sparkle updater controller optional and only construct it when running from a .app bundle that declares SUFeedURL; hide the "Check for Updates" menu item and log a warning on manual checks otherwise
• Document the override, its constraints, and the GUI-launch environment caveat in macos/README.md

Key Implementation Details

• resolve is guarded by #if DEBUG and returns nil unconditionally in release builds, so a shipped app cannot be redirected to an arbitrary binary through the environment.
• An unset or blank variable is ignored; a value that is relative, missing, a directory, or a non-executable file throws ForgeCoreError.missingExecutable so typos fail loudly instead of silently falling back to the downloaded runtime. wrapIfOverridden logs that error and returns the base installer.
• The override reports version 0.0.0 so it never sorts as an upgrade, and the existing launch-boundary checks in RuntimePinnedExecutable still apply to the overridden path.
• Skipping Sparkle unbundled avoids the crash path where Sparkle's configuration failure NSAlert faults inside ImageIO; packaged builds keep the previous behaviour.

Testing

Run the tests added in macos/Tests/ForgeMenuCoreTests/CoreBehaviorTests.swift, which cover override resolution under DEBUG, the release-build nil result, empty/unset variables, and rejection of relative, missing, directory, and non-executable paths. Manually verify with FORGE_RUNTIME_BINARY=/path/to/forge3 .build/debug/ForgeMenuBar and confirm the unbundled debug binary launches without an updater menu item.