feat(config): overhaul product manifest schema + 51 tests
- Redesigned schema to match real-world product.json files across ecosystem
- Changed 'id' → 'productId' (matches all existing files)
- Support bundleId as string OR per-platform object
- Added backendPort, tagline, primarySurface, appStore, bundleIds fields
- Added legacy identity fields (licensePrefix, configDirName, envVarPrefix, etc.)
- Added duplicate container name validation via superRefine
- Fixed loadProductManifestSync: require() → readFileSync (ESM-safe)
- Added BundleIdSchema and AppStoreSchema exports
- Added 'mac' platform option
- 51 new tests covering all schemas, validation, file loading, real-world manifests
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